Participant Experts in Ontario Personal Injury Litigation: Rule 53.03 After Westerhof

Treating professionals have significant first hand evidence in personal injury actions. This evidence cannot easily be recreated by an expert retained years after an injury. A family doctor could have assessed the plaintiff within days of an accident. A psychologist may have observed the development of symptoms over several years. A physiotherapist or occupational therapist may have recorded changes in function that would not be apparent during a single medical-legal examination.

Ontario law permits some of those professionals to give opinion evidence without serving a report that complies with Rule 53.03 of the Rules of Civil Procedure. The exception is important, but it is narrower than the familiar shorthand that treating physicians do not need expert reports.

A Court will want to know specifics of the particular opinion such as: when it was formed, why it was formed, what information produced it and whether it arose through the witness’s participation in the events underlying the litigation.

Under Westerhof, treating professionals are enable to provide opinion evidence without a Rule 53.03 report only if the opinion arose from the witness’s observation of or participation in the events at issue and was formed through the ordinary exercise of professional expertise at that time.

Since Westerhof v. Gee Estate, 2015 ONCA 206, Ontario courts have repeatedly returned to adjudicating opinions that lay along that boundary. Imeson v. Maryvale addressed a clinician who had moved beyond patient-specific treatment into liability and causation evidence. Taylor v. Zents confirmed that a treating professional may also qualify as a litigation expert. Two mid-trial rulings in Wallis v. Wawanesa then showed that the litigation-expert component can be withdrawn while the witness remains entitled to give participant-expert evidence.

In serious personal injury litigation, Ontario personal injury lawyers must distinguish opinions formed through treatment from opinions later developed to prove the lawsuit. The consequence of not having valuable treating opinions permitted into evidence can be devastating for plaintiffs at trial.

What Westerhof Exempts

Rule 53.03 of the Rules of Civil Procedure requires that a party who intends to call a litigation expert at trial serve a signed report containing prescribed information.

The report must identify the expert’s name, address, area of expertise, qualifications, relevant employment and educational experience, instructions, opinions, reasons, assumptions, research and the documents relied upon. It must also include the expert’s signed acknowledgment of the duty to provide evidence that is fair, objective and non-partisan.

The current rule further requires a signed certification addressing the authenticity of authorities, documents and records referred to in the report, together with details of specified doubts about authenticity. The authenticity-certification requirements in paragraphs 6.1 and 6.2 of Rule 53.03 took effect on December 1, 2024 and remain part of the current Rules.

Westerhof held that this formal report regime does not apply to every witness who possesses specialized knowledge and gives an opinion.

Witnesses who were not engaged by or on behalf of a party to provide evidence in the litigation can still provide an opinion for its truth without complying with Rule 53.03 if:

1. the opinion is based on the witness’s observation of or participation in the events at issue; and

   
   

2. the opinion was formed through the ordinary exercise of the witness’s skill, knowledge, training and experience while observing or participating in those events.

Treating professionals can often meet these requirements. The exception can also apply to a non-party expert who formed an opinion through an examination or investigation undertaken for a purpose other than the tort action. An example of such a non-party expert would be a physician who performed a section 44 assessment on behalf of an accident benefits insurer.

The source and purpose of the opinion are crucial. A physician who diagnosed a condition while treating the plaintiff is not in the same position as a physician was retained later to review the treatment file and provide an opinion for trial.

Westerhof also outlined the limits of this exemption. The Court elucidated that if a participant or non-party expert proposes to give an opinion extending beyond what was formed through the relevant observation, treatment or assessment, Rule 53.03 applies to that additional opinion.

The Inquiry Is Opinion-Specific

A witness does not acquire one status that governs everything they may say at trial. For example, a treating psychologist could recount the plaintiff’s reported symptoms and the treatment provided as a fact witness. The psychologist may render a diagnosis or prognosis that was formed during treatment as a participant expert. If the psychologist later reviews surveillance, defence reports and a medical brief before preparing an opinion for counsel about legal causation or employability, that opinion will be governed by Rule 53.03.

The same witness can therefore give fact evidence, participant-expert evidence and litigation-expert evidence.

Personal injury lawyers in Ontario must examine the nature of the opinion sought to be provided as opposed to categorizing the witness. That approach avoids two common errors. The first is assuming that anything said by a treating professional falls within Westerhof. The second is assuming that a professional who prepares a Rule 53.03 report loses the ability to provide participant evidence arising from treatment.

Recent Ontario decisions have rejected both of those propositions.

When Was the Opinion Formed?

Timing is important, although not necessarily conclusive. Opinions which are recorded contemporaneously in the clinical chart will usually be easier to characterize as one formed during treatment. The notes may show that the professional diagnosed a condition, considered its likely cause, imposed restrictions or made a referral because of a prognosis reached at the time.

However, a participant expert is not invariably confined to opinions written verbatim in the chart.

That issue arose in Wallis v. Wawanesa Mutual Insurance Company, 2026 ONSC 2423. The treating psychologist’s notes did not consistently record the precise diagnosis or prognosis she proposed to give at trial. The defence argued that participant evidence should be limited to opinions appearing in the contemporaneous records.

The court rejected an absolute writing requirement. An opinion may have been formed during treatment without having been fully recorded. The witness must still establish that the opinion existed at the relevant time, and the records can be used to refresh the witness’s memory.

The absence of a contemporaneous entry remains significant. Clinical professionals prepare records for treatment, continuity of care and regulatory purposes rather than for a later evidentiary dispute. A sparse chart does not prove that no professional judgment was formed, can certainly make timing and content of the proposed opinion more challenging to establish.

It is also important to keep in mind that a later narrative letter is not automatically a litigation-expert report. Counsel could ask a treating professional to summarize a diagnosis, prognosis or restriction previously formed during treatment. Putting an existing opinion into a more accessible format does not necessarily change its character.

The position changes if counsel asks the professional to perform new work. Reviewing additional records, reconciling competing expert reports or addressing an issue never considered during treatment is more likely to be considered a Rule 53 litigation opinion. Simply nameing the resulting document a treatment summary does not bring it within Westerhof.

Causation Requires Particular Care

Causation is a particularly tricky area for plaintiff lawyers to navigate with treating experts. Courts have not applied a general rule that treating professionals may always address it or may never address it. However, it is less likely that causation will be explicitly opined upon within a treating expert's clinical notes.

Clinicians do certainly sometimes consider causation while providing treatment. For example, an emergency physician may diagnose a fracture as traumatic. A neurologist may assess whether the onset of symptoms is consistent with a recent head injury. A psychologist may consider the relationship between an accident and a psychological condition when developing a treatment plan.

Where a causation opinion was genuinely formed for a treatment or assessment purpose, within the professional’s expertise and through participation in the relevant events, the subject of causation does not by itself determine whether Rule 53.03 applies.

The difficulty is that a clinical opinion may rest on a history accepted for treatment rather than an independent determination of disputed facts. A professional could explain the assumptions on which a diagnosis or treatment decision was based without being entitled to prove that every assumed event occurred.

Imeson v. Maryvale (Maryvale Adolescent and Family Services), 2018 ONCA 888 illustrates that distinction.

In that case, plaintiff had alleged that he was sexually assaulted while residing at Maryvale. His treating clinician gave evidence linking later psychological difficulties to the alleged assaults and addressed problems commonly experienced by survivors of childhood sexual abuse.

The Court of Appeal held that portions of the evidence exceeded the proper scope of participant-expert testimony. The clinician had accepted the plaintiff’s account for therapeutic purposes, but treatment did not place him in an appropriate position to decide whether the assaults occurred. His generalized evidence about survivors as a group was not based on his observation or treatment of this particular plaintiff.

Imeson does not mean that every causation opinion falls outside Westerhof. It prevents a participant expert from using therapeutic acceptance of a disputed history as proof of the underlying event or moving from patient-specific treatment into broader analysis that was not formed through that treatment.

Counsel should separate three propositions:

• the history the patient provided;

• the professional opinion formed on the assumption that the history was accurate; and

• proof that the disputed history was true.

Treating professionals might be able to testify about the second without establishing the third.

The first Wallis ruling should also be read carefully. The psychologist was initially permitted to address causation because the court allowed her to act in a dual capacity and found that her three narrative reports, read together, sufficiently complied with Rule 53.03. After her litigation-expert qualification was withdrawn, the participant evidence expressly preserved by the court concerned diagnosis, prognosis and treatment recommendations formed during treatment.

Reviewing Litigation Material Is Not Participation

Someone does not become a participant expert merely because they were retained before an action began or reviewed records connected with the incident.

In Snodden v. 2568832 Ontario Inc., 2023 ONSC 6759, the proposed opinion concerned the cause of a fire. The witness relied on photographs and information generated by others rather than attending the scene and forming an opinion through personal involvement in the relevant events. The work was characteristic of a conventional litigation expert, not a participant expert.

A similar distinction was drawn in Village Homes Inc. v. Connelly, 2025 ONSC 1946. The proposed participant expert was permitted to provide the Court with an opinion about cracking he had personally observed at the site. He was not permitted to give an opinion about the alleged absence of rebar and concrete where that conclusion was based on a demolition video and information supplied by someone else.

The result was opinion-specific. Some of the witness’s evidence arose from direct participation; some did not.

The same reasoning applies in personal injury litigation. A treating physician may give opinions formed through examining the plaintiff. If the physician is later supplied with surveillance, discovery transcripts, engineering reports or a large medical brief, opinions newly developed from that material may require Rule 53.03 compliance.

A Treating Professional May Also Be a Litigation Expert

A therapeutic relationship does not inherently disqualify a professional from serving as a litigation expert. The Court of Appeal addressed the issue directly in Taylor v. Zents, 2025 ONCA 662.

The plaintiff’s treating psychologist testified as a participant expert and was also qualified under Rule 53.03. The defendant argued that the psychologist could not be impartial because she had treated the plaintiff for more than five years and occupied a supportive therapeutic role.

The Court of Appeal upheld the qualification.

Participant experts can also qualify as a Rule 53.03 expert if the court is satisfied that the relationship with the plaintiff does not prevent the witness from fulfilling the duty to provide fair, impartial and objective assistance.

The question is not whether a reasonable observer might perceive the witness as lacking independence. It is whether the relationship or interest leaves the expert unable or unwilling to carry out the primary duty owed to the court.

Once the expert had acknowledged their duty to the Court, the party opposing the evidence must show a realistic concern that the expert cannot or will not comply with it. If that threshold is met, the burden shifts to the proponent to establish impartiality on a balance of probabilities.

The trial judge in Taylor considered the psychologist’s evidence during the qualification voir dire, the content and tone of her report and whether she appeared to have acted as an advocate. The Court of Appeal found no error in the conclusion that she remained capable of giving objective evidence.

The availability of an expert without a treatment relationship did not itself require exclusion. In the absence of a realistic concern about impartiality, the plaintiff was entitled to present the case through the treating psychologist in both capacities.

Taylor does not mean that dual qualification should be assumed. It confirms that the therapeutic relationship is relevant but not determinative.

The Two Wallis Rulings

The two 2026 Wallis decisions demonstrate how the dual-role analysis may change as the evidence unfolds.

In Wallis, the plaintiff sought to call her treating psychologist of approximately nine years. The psychologist’s clinical records were already in evidence, and she had prepared three narrative reports containing opinions extending beyond the ordinary participant role.

In the first ruling, Wallis v. Wawanesa Mutual Insurance Company, 2026 ONSC 2423, the court found that the reports, considered collectively, sufficiently complied with Rule 53.03. The defendant had received substantial notice of the proposed opinions, and its experts had an opportunity to respond.

Relying in part on Taylor, the court also declined to treat the longstanding treatment relationship as an automatic bar to litigation-expert qualification. The psychologist was initially permitted to give participant evidence and broader litigation opinions, subject to the court’s continuing control over admissibility.

The position changed after cross-examination.

However in a later trial ruling, Wallis v. Wawanesa Mutual Insurance Company, 2026 ONSC 2636, the court revisited the qualification. The first ruling had proceeded on the understanding that the psychologist reviewed a comprehensive medical brief before preparing her broader opinions. Her testimony revealed that she had just reviewed an index rather than the actual underlying records, together with a more limited group of reports.

The court was unable to determine with sufficient clarity what documentation she had reviewed and when she reviewed it. Although the reports appeared even-handed in tone, the uncertainty surrounding their factual foundation prevented the court from treating the broader evidence as objective in the sense required of a litigation expert.

The psychologist was disqualified from continuing in the role of a rule 53 expert, and the jury was instructed to disregard the evidence that depended on the litigation-expert qualification.

The Court did not exclude altogether. The court preserved her participant-expert evidence concerning diagnosis, prognosis and treatment recommendations formed during treatment. It also found that many opinions contained in the three reports remained admissible within that narrower capacity.

Read together, the rulings demonstrate that:

• one professional may occupy both roles;

• a later report can contain both existing treatment opinions and newly developed litigation opinions;

• substantial compliance and adequate notice do not end the admissibility inquiry;

• the court’s gatekeeping obligation continues after qualification;

• the litigation-expert portion of the testimony may be excluded without eliminating the participant evidence; and

• counsel must be able to identify the foundation and chronology of each broader opinion.

The second ruling is a trial-level application, not a change to the appellate test in Westerhof or Taylor. Its importance lies in showing how those principles can operate during a personal injury trial.

Rule 53.03 Compliance Does Not Resolve Admissibility

A report containing the information prescribed by Rule 53.03 does not guarantee that the opinion will be admitted.

The witness must possess the necessary expertise. The evidence must satisfy the ordinary requirements governing expert opinion, including relevance and necessity. The opinion must rest on a sufficiently reliable factual foundation, and the expert must be willing and able to provide fair, objective and non-partisan assistance.

The court must then weigh the potential benefit of the evidence against its risks. Those risks can include unfair prejudice, confusion, excessive consumption of trial time and the possibility that the trier of fact will assign undue weight to the expert’s status.

In Bruff-Murphy v. Gunawardena, 2017 ONCA 502, the Court of Appeal emphasized that the trial judge’s gatekeeping obligation continues after an expert has been qualified. Evidence may have to be restricted or excluded if concerns about impartiality, reliability or admissibility become apparent during the testimony.

The Court of Appeal reinforced that principle in Pederson v. Forget, 2026 ONCA 118.

Pederson was an occupiers’ liability case rather than a participant-expert case. The plaintiff’s expert had conducted slip-resistance testing on stairs after the incident. The trial judge excluded the proposed testing evidence because the conditions under which it was performed did not correspond sufficiently with the conditions surrounding the fall.

The Court of Appeal upheld that decision. Qualifying a witness in an area of expertise does not make every opinion, experiment or methodology offered by the witness admissible. The trial judge remains required to assess the reliability and probative value of the particular evidence and weigh it against the potential prejudice.

For a dual-role witness, the court may therefore have to identify which opinions remain admissible through participation and which depend on the Rule 53.03 qualification. The loss of the latter does not necessarily require exclusion of the former.

Disclosure Still Has to Be Meaningful

The exemption from Rule 53.03 is not an exemption from fair notice. Treating professionals will often prepare notes, consultation reports and correspondence that disclose the opinions formed during treatment. Those records may provide opposing counsel with sufficient notice of the proposed evidence.

Producing thousands of pages of clinical records and stating that a physician may testify about treatment does not necessarily identify the diagnoses, restrictions, causation opinions or prognoses that will be advanced for their truth. Where the intended opinion is not apparent, the opposing party may seek clarification through documentary discovery, examinations, a will-say statement or a pretrial ruling.

A focused summary of opinions already formed may reduce uncertainty without turning those opinions into new litigation work. Care is required in obtaining it. Asking a professional to identify an existing diagnosis is different from asking the professional to review new evidence and reach a conclusion for counsel.

If a dual role is intended, the distinction should be clear in the reports and proposed evidence. The opinions said to arise from treatment should be separated from the additional Rule 53.03 opinions. Instructions, factual assumptions and documents relied upon should be identified with enough precision to avoid the uncertainty that arose in the second Wallis ruling.

Participant Evidence and Section 52 Reports Are Separate Routes

Westerhof addresses when a witness may give opinion evidence without serving a Rule 53.03 report. It does not provide the only route through which a medical opinion may be admitted.

Section 52 of Ontario’s Evidence Act permits a signed report obtained by or prepared for a party by a qualified practitioner to be admitted with leave of the court after at least ten days’ notice has been given to the other parties.

Reports which are properly admitted under section 52 can be received for the truth of the medical opinions it contains. At the request of an opposing party, the trial judge must require the practitioner to attend so that the report can be tested through cross-examination. If the practitioner is not made available following that request, the opinion cannot be admitted for its truth.

That statutory route differs from the business-record provision in section 35. Section 35 can permit admission of records made in the usual and ordinary course of business, but it does not by itself make medical opinions concerning diagnosis or causation admissible for their truth merely because they appear in a clinical chart.

The Court of Appeal explained the distinction in Girao v. Cunningham, 2020 ONCA 260. The trial court had to distinguish between records admitted for a limited business-record purpose and practitioner reports tendered under section 52 as substantive medical opinion evidence. A party relying on section 52 must also respect the opposing party’s ability to test the opinion through cross-examination.

Counsel should therefore distinguish among:

• the admission of a clinical record under section 35;

• the admission of a practitioner’s report under section 52;

• oral participant-expert evidence under Westerhof; and

• litigation-expert evidence governed by Rule 53.03.

Defining the Witness Before Trial

Participant-expert disputes are easier to manage before the witness enters the courtroom. Counsel should identify each opinion to be advanced, when it was formed and where it is documented. The professional should be asked whether it arose during treatment, during an assessment conducted for another purpose or only after counsel requested further analysis.

Any later work should be separated from the earlier clinical role. If the professional reviewed additional records, competing reports, discovery transcripts or surveillance, counsel should determine whether that work generated a new opinion requiring Rule 53.03 compliance.

The proposed area of expertise also requires care. Participant status does not permit a witness to testify beyond the field in which the witness is qualified. A family physician does not acquire expertise in vocational economics. A physiotherapist is not necessarily qualified to determine the neurological cause of every limitation. A psychologist who accepted a history for treatment is not thereby qualified to decide whether a disputed accident or assault occurred.

Conclusion

Westerhof did not establish a broad treating-physician exemption from Ontario’s expert-report rules. It recognized that professionals may form relevant opinions through their ordinary involvement in events that later become the subject of litigation. Those opinions may be admitted without a Rule 53.03 report because they were not created to prove the action.

The exception follows the opinion, not the witness. Participant experts can provide an opinion formed through direct observation or participation and through the ordinary exercise of professional expertise. The witness cannot use participant status to introduce an analysis later developed from a litigation brief, move beyond the witness’s expertise or prove disputed events that were simply accepted as part of a therapeutic history.

A treating professional may also qualify as a litigation expert. Taylor v. Zents confirms that the therapeutic relationship is not an automatic disqualification. The two Wallis rulings show why the distinction between the roles remains important: a court may initially permit the dual role, later exclude the litigation-expert evidence and still allow the witness to testify about opinions formed through treatment.

Rule 53.03 compliance is also only one part of the analysis. Bruff-Murphy and Pederson confirm that the court continues to scrutinize the reliability, foundation and potential prejudice of each opinion after the witness has been qualified. Section 52 of the Evidence Act creates a separate route for practitioner reports, with its own notice, leave and cross-examination requirements.

In serious personal injury litigation, these boundaries may determine whether evidence concerning diagnosis, causation, prognosis, employability and future treatment reaches the trier of fact. They should be addressed when the witness and proposed opinions are identified, not for the first time during an objection partway through trial.

Foster Injury Law acts in serious personal injury cases across Ontario, including claims in which treating professionals and other participant experts provide central evidence concerning the nature and consequences of an injury.