The Ontario Multi-Centre Exercise-Heart trial is making a prospective study of 751 male subjects following well-documented episodes of myocardial infarction. Comparison is here made between the 50 participants who sustained a recurrence of their myocardial infarction, and the 701 participants who did not. Reinfarction was a little more likely with a history of multiple previous infarctions, but was unrelated to such indicators of infarct severity as symptoms, electrocardiographic abnormalities, enzyme changes, cardiac arrest, arrhythmia, or minimum systolic blood pressure. Features noted on admission to the trial, which may have an adverse effect, include smoking history and related symptoms, residual disability, shortness of breath, and angina of effort. The main physiological warning sign was a low and decreasing cardiac output at a submaximal work load, with a compensatory widening of arteriovenous oxygen difference. With the possible exception of exercise non-compliance, none of the adverse findings is sufficiently consistent to be of value when advising individual patients.