Can I recover damages for a lost chance in a professional negligence claim in Ontario?

It's difficult, and courts approach these claims cautiously. Ontario generally requires you to prove causation on a balance of probabilities - meaning you show it's more likely than not that the professional's negligence caused your actual loss, not merely that it cost you some chance of a better outcome. Pure "loss of chance" claims, where a plaintiff asks to be compensated for a reduced probability of success rather than a proven loss, have had a limited and inconsistent reception in Canadian courts outside a few specific contexts.

Where this issue comes up most often is legal malpractice involving a lost claim or a missed limitation period - there, courts do sometimes assess damages by evaluating what the underlying claim was probably worth, including a realistic discount for litigation risk the client would have faced anyway even with a competent lawyer. That's different from awarding damages purely for a lost statistical chance. Because this area is fact-sensitive and the legal framework is not settled the same way across every professional context, a claim resting mainly on lost chance rather than provable causation is a real litigation risk that should be assessed carefully before proceeding.