The Basic Two-Year Limitation Period
Under Ontario’s Limitations Act, 2002, most civil claims are subject to a basic two-year limitation period[1]. Section 4 of the Act provides that a legal action cannot be commenced after the second anniversary of the date on which the claim was discovered. In practical terms, this means a plaintiff generally has two years to sue from the time they discover that they have a claim[2]. If a lawsuit is filed after this two-year window, the defendant can raise a limitations defence, arguing the claim is statute-barred (out of time).
Discoverability: When a Claim Is Discovered
The concept of discoverability is crucial in determining when the limitation “clock” starts ticking. Ontario’s Limitations Act codifies the common-law discoverability rule in section 5(1). A claim is considered “discovered” on the earlier of two dates[3]:
- Actual Knowledge (s.5(1)(a)): The day when the person with the claim first knew all of the following four facts: (i) that an injury, loss or damage has occurred; (ii) that the injury, loss or damage was caused by or contributed to by an act or omission; (iii) that the act or omission was committed by the person against whom the claim is made (i.e. the potential defendant); and (iv) that, given the nature of the harm, a legal proceeding is an appropriate means to seek a remedy[4].
- Deemed Discovery (s.5(1)(b)): The day on which a reasonable person in the claimant’s circumstances ought to have known all of the above matters[5]. This is an objective test to capture when the claim should have been discovered with reasonable diligence.
Importantly, the law presumes that a claimant knew of these matters on the day the act or omission occurred unless the contrary is proven[6]. In other words, there is a rebuttable presumption that the clock starts at the time of the wrongdoing. The plaintiff can rebut this by showing that they neither knew nor reasonably could have known of the claim until a later date.
The Doctrine of Discoverability in Practice
The doctrine of discoverability means that the limitation period does not necessarily start on the date of the negligent act or injury itself, but rather when the plaintiff becomes aware (or ought to have become aware) of the key facts giving rise to the claim[4]. In many cases, the injury and its cause are immediately apparent, so the cause of action is discovered right away. However, in other cases – especially those involving delayed diagnosis, misdiagnosis, or ongoing professional relationships – the injured party may not immediately realize that they have been harmed by someone’s negligence.
Ontario courts have consistently recognized that for delayed discovery scenarios, the clock begins when a reasonable person would know of the injury and its likely cause in negligence, not necessarily when the negligent act occurred[7][8]. This principle protects plaintiffs who were diligent but unaware of their injury or its cause due to latent harm or the actions of the defendant.
Reliance on Professionals and “Appropriate Means” to Sue
A common situation where discoverability delays the start of the limitation period is in cases of professional negligence, such as medical or dental malpractice. Often, a patient or client might suspect something is wrong but is reassured by the professional that there is no issue, or the professional continues to attempt to remedy the problem. In such situations, the patient may reasonably rely on the professional’s expertise and delay suing while corrective efforts are ongoing[9][10]. Ontario courts have held that it would not be appropriate to expect a layperson to commence litigation during the period the trusted professional is actively trying to fix the mistake or alleviate the harm[9]. This ties into the fourth element of discovery (s.5(1)(a)(iv)): a claim isn’t fully “discovered” until the plaintiff knows that a legal proceeding is an appropriate means to seek a remedy[8]. If the plaintiff reasonably believes the issue is being resolved without litigation – for example, through continued treatment or repair by the professional – then it may not yet be appropriate to sue.
The Ontario Court of Appeal has emphasized that a plaintiff’s reliance on a defendant’s superior knowledge and assurances can toll (pause) the limitation clock[10]. This principle was applied in a series of cases involving ongoing professional relationships: – Brown v. Baum (2016 ONCA 325): A patient underwent surgery and suffered complications, but her surgeon continued to treat and perform corrective surgeries for over a year after the initial operation. The patient did not sue until nearly two years after the last corrective procedure (and over three years after the original surgery). The court found the claim was not late – even though the injury occurred earlier – because the patient did not know (and no reasonable person would conclude) that suing was appropriate while the doctor was still attempting to remedy the issue[8][9]. Essentially, the limitation period did not commence until the corrective treatment ended, as that was when the patient could first reasonably perceive that negligence had occurred and that legal action might be necessary. The Court of Appeal approved this approach, noting it would be unfair to force a patient to sue a doctor who might still successfully fix the problem[9]. Had the patient sued immediately, the doctor likely would have ceased providing treatment, which is a strong policy reason to delay the clock[11].
– Chelli-Greco v. Rizk (2016 ONCA 489): In this dental negligence case, a patient had problems with a dental bridge and even threatened legal action when it failed. The dentist denied any fault and promised to “endeavor to repair” the issue, continuing to treat the patient for some time[12][7]. The patient ultimately terminated the dentist-patient relationship and sued within two years of that termination. The court held the claim was only discovered once the professional relationship ended and the patient realized the dentist’s efforts were unsuccessful[7]. Although the initial mishap with the bridge occurred earlier, the patient had been led to believe the problem was being fixed and thus did not know that litigation was necessary. The Ontario Court of Appeal upheld this finding, confirming that the limitation period was effectively tolled during the period the dentist was attempting to remediate the harm[7][13]. This case illustrates that in many medical or dental malpractice situations, the clock starts not at the moment of the error, but when the patient stops reasonably relying on the professional’s reassurance or attempts at correction[14].
These decisions align with a broader trend in Ontario law: the “appropriate means” doctrine (from s.5(1)(a)(iv)) has been used to create fairness in limitation period assessments[15][10]. If a defendant’s conduct effectively delays the plaintiff’s realization that they have a legal claim – for example, by continuously assuring the plaintiff that the issue is being handled – courts may find that the limitation period does not run during that time[10]. The result is that plaintiffs are not penalized for giving the professional a chance to set things right, and defendants cannot exploit the time they kept the plaintiff in the dark to avoid liability.
Delayed Diagnosis and Misdiagnosis Cases
In cases of delayed diagnosis or misdiagnosis (a subset of medical negligence), the injury often isn’t apparent until later diagnostic tests or second opinions reveal the true situation. Ontario courts acknowledge that the limitation period in such cases begins when the patient, exercising reasonable diligence, learns of the misdiagnosis and the harm, and appreciates that it was likely caused by an error in the initial treatment[10]. It does not necessarily begin on the date the medical error actually happened, because the patient may have had no reason to suspect malpractice at that time[7]. For example, a person might receive negligent treatment or a wrong medication dosage but feel only mild symptoms initially. If their doctor repeatedly insists nothing is wrong, the patient might not discover the negligence until months later when a new doctor or test exposes the error. In such a scenario, the cause of action (the legal claim) is “discovered” only at that later point of realization, not on the date of the original medical mistake.
Ontario jurisprudence, including the cases mentioned above, supports this approach: the clock starts ticking when a reasonable person in the plaintiff’s position would have known of the injury and that it was caused by potential negligence[7][9]. In other words, the law asks: “When should the claimant have known that something went wrong with the treatment, and that it was serious enough that suing the practitioner would be appropriate?” If the answer is a date later than the incident, that later date is when the two-year limitation begins.
This principle was affirmed in Thompson v. Sehgal (2021 ONSC 5944), a medical negligence case involving delayed discovery of harm. The court in Thompson reiterated that certainty about the full extent of the injury or the professional’s negligence is not required to start the clock; rather, the clock starts when the plaintiff has knowledge of the material facts that would lead a reasonable person to suspect negligence[7]. If the plaintiff was unaware due to the defendant’s reassurances or the latent nature of the injury, the limitation period is extended until discovery. This ensures that patients who discover a misdiagnosis later on (despite earlier symptoms or issues) can still seek justice within two years of uncovering the error.
Outcome and Importance for Limitation Defence
When the doctrine of discoverability is successfully invoked, a lawsuit that might appear out-of-time can actually be deemed timely. In the context of a patient who learns of a medical/dental injury well after the treatment, the two-year limitation window runs from the point of discovery, not the date of the treatment. As long as the claim is filed within two years of when the plaintiff discovered (or ought to have discovered) the negligence, it will fall “well within the statutory two-year period.” In our example scenario, if the patient only uncovered the negligent cause of her injury by early 2023 and filed the lawsuit by mid-2024, the action would be comfortably inside the two-year limit. The limitations defence would likely fail in such circumstances, because the defendant cannot show the claim was discovered more than two years before it was commenced.
In summary, the Limitations Act’s discoverability provisions and Ontario case law make clear that a defendant’s attempt to dismiss a claim as statute-barred will be unsuccessful if the plaintiff did not know and could not reasonably have known of the claim until a later date. Especially in cases of medical malpractice and other professional negligence, courts are unwilling to cut off plaintiffs’ rights where the plaintiffs were diligent but understandably unaware of the wrongdoing for some time. The policy underlying this is fairness: people should not lose their chance to sue before they even realize they’ve been wronged. Thus, the doctrine of discoverability serves as a vital protection for plaintiffs, ensuring that the limitation period only starts when a claim truly comes to light[3][10].
Sources:
- Limitations Act, 2002, S.O. 2002, c. 24, Sched. B (s. 4 and s. 5(1))[3][6]
- Brown v. Baum, 2016 ONCA 325 – Court of Appeal for Ontario (medical malpractice; clock tolled until end of treatment)[8][9]
- Chelli-Greco v. Rizk, 2016 ONCA 489 – Court of Appeal for Ontario (dental negligence; claim discovered when dentist-patient relationship ended)[7][14]
- Thompson v. Sehgal, 2021 ONSC 5944 – Ontario Superior Court of Justice (delayed diagnosis; limitation period begins at discovery of negligence)[7]
- Ontario Court of Appeal in Presidential MSH Corp. v. Marr Foster & Co. LLP, 2017 ONCA 325 (and related cases) – discussing reliance on a professional’s expertise and the “appropriate means” element of discoverability[10].
- Torys LLP, The uncertainty behind Ontario’s limitation periods (2019) – analysis of recent trends in discoverability and the appropriate means doctrine[10].
[1] [3] [4] [5] [6] Limitation periods in Civil Litigation Claims | Richardson Hall LLP
[2] When does the Limitation Period Start in a Lawsuit in Ontario?
https://www.gluckstein.com/news-item/delayed-discoverability-when-does-the-limitation-period-start
[7] [12] [13] [14] Limitations Extended in Dental Professional Negligence
[8] [9] [11] Discoverability Principal Applied to Medical Malpractice Case
[10] [15] The uncertainty behind Ontario’s limitation periods | Insights | Torys LLP