This study enrolled 651 men with myocardial infarction in five participating centers in a randomized 3 year clinical trial of the effects of prescribed supervised exercise. The subjects, aged to 30 to 64 years, were screened for eligibility 2 to 36 months after their qualifying myocardial infarction. The men in the exercise group pursued intensive exercise in the laboratory for 8 weeks and then in a gymnasium for 34 months. The experience of the exercise group was more favorable than that of the control group in most of the comparisons made. The cumulative 3 year total mortality rate was 7.3 percent for the control group and 4.6 percent for the exercise group; the 3 year rate for recurrent myocardial infarction was 7.0 and 5.3 percent, respectively. Mortality rates in the two groups did not differ significantly, but the data were consistent with an assumption of substantial benefit from exercise. Adjustment for small differences in baseline variables by multivariate methods did not materially alter the estimate of effect of exercise. Certain subgroups showed a greater benefit from exercise.