The Court of Appeal for Ontario recently released its reasons in Westerhof v Gee Estate,1 which hinged on whether the opinion evidence of medical and other practitioners involved in treating an individual should be subject to the requirements for expert witnesses set out in the Rules of Civil Procedure.

The Court of Appeal, clarifying inconsistent prior law, held that the evidence of these “participant experts” should not be subject to the rules related to expert witnesses.


The Westerhof appeal related to two conflicting Superior Court of Justice decisions – Westerhof v Gee Estate and McCallum v Baker. Both cases were actions in tort stemming from motor vehicle accidents that turned on the application of the rules in respect of expert witnesses. In particular, these cases engaged the expert’s duty to the court,2  and the formal evidentiary requirements governing the submission of expert reports.3

In Westerhof, the plaintiff sought to adduce evidence from a number of doctors and other witnesses concerning history, diagnosis and prognosis in relation to his alleged injuries. As the Divisional Court explained, “A substantial amount of the medical evidence the plaintiff intended to call was not admitted by the trial judge either because the witness was not qualified to provide it or because the requirements of rule 53.03 had not been complied with.”4  The plaintiff in Westerhofappealed the trial judge’s decision to rule this evidence inadmissible.

In McCallum, the trial judge allowed several medical practitioners who had treated the injured plaintiff to give opinion evidence concerning the plaintiff’s “future employment prospects and future treatment needs.” The trial judge reasoned that this evidence was admissible without complying with the formal requirements related to expert testimony because these individuals had been directly involved in treating the plaintiff.5 The defendant in McCallum appealed the decision, claiming that this evidence should have been ruled inadmissible for not complying with the rules.

The Divisional Court in Westerhof held that the key consideration in determining whether to admit opinion evidence of individuals involved in treating the plaintiff was “not in the role or involvement of the witness, but in the type of evidence sought to be admitted.”6 Simply put, if the evidence to be tendered is opinion evidence, then the rules regarding expert testimony must be followed.7

Court of Appeal rules that “participant experts” not bound by expert rules

The Court of Appeal rejected the Divisional Court’s reasoning that the key determination in evaluating admissibility of evidence (where rule 53.03 has not been complied with) is whether the evidence at issue is opinion evidence.8

In its reasons, the Court of Appeal considered at length the obligations imposed on expert witnesses in the rules and the reasons behind those obligations, as described in the Osborne Report.9

Its analysis of the purpose behind the rules regarding experts, and the law as it stood prior to the implementation of those rules, led the court to hold that, “A witness with special skill, knowledge, training, or experience who has not been engaged by or on behalf of a party to the litigation may give opinion evidence for the truth of its contents without complying with rule 53.03 where:

  • the opinion to be given is based on the witness’s observation of or participation in the events at issue; and
  • the witness formed the opinion to be given as part of the ordinary exercise of his or her skill, knowledge, training and experience while observing or participating in such events.”10

If the participant expert proffers opinion evidence extended beyond these limits – namely the opinions formed during participation in the events of the litigation – he or she will have to comply with rule 53.03.11

The court rejected the holding of the Divisional Court for a number of reasons, including the following findings:

  • the introduction of rules governing the conduct of experts was intended, among other things, to limit the practice of litigants using “hired guns” or “opinions for sale” in civil litigation, which practice tended to lead to biased expert evidence being given at trial. The involvement of participant experts in litigation, who are presumably less inclined to advocate for a party’s position, likely does not engage these issues of bias and would not appear to have been a focus of the Osborne Report;12  
  • the text of rule 53.03, which states that it applies to any “expert engaged by or on behalf of a party to provide [opinion] evidence in relation to a proceeding,” would not apply to participant experts who are not typically retained by any party to litigation;13  and
  • requiring application of rules related to experts to participant experts would add to the cost of and delay litigation in significant and unnecessary ways.14

Impact – a wider scope for admissibility of expert opinion

The Court of Appeal’s reasons in Westerhof broaden the scope of evidence that may be tendered by participation experts in civil proceedings without compliance with rules regarding experts. Individuals who are personally involved in events giving rise to litigation and form opinions based on that involvement will be able to testify to those opinions, without being required to comply with the rules that would otherwise ensure that (a) they are properly qualified as expert witnesses and (b) they act as an objective and unbiased friend to the court.

While the Westerhof ruling concerns (mostly) medical practitioners in the personal injury context, the Court of Appeal’s reasons will apply to civil litigants in any setting (including accounting, engineering, finance, etc.). The question of precisely how much weight the courts should give these participant expert opinions – possibly in the face of opinions from qualified experts in compliance with the rules – remains to be determined in the circumstances of each case.