Seventy-six survivors of a prior myocardial infarction were subjected to maximal treadmill stress testing. At least two months had elapsed between the acute episode and the inclusion of the patients in the study. In 39% of the cases the infarction was located to the inferior wall of the left ventricle, in 42% it was anterior and in 18% it was both anterior and inferior. The stress test was positive in 51% and negative in 46% of the cases, while it was non-diagnostic in 3%. The high percentage of negative tests may be explained by the fact that, in these patients, the coronary obstructive lesions are limited to the vessels supplying infarcted myocardium, while the remaining muscle has an adequate coronary perfusion. The percentage of positive tests was higher in patients with anterior (59%) than in those with inferior wall infarctions (43%). In the group of patients with anteroapical aneurysms the incidence of ischemic responses to exercise was 73%. In two of these cases, there was an exercise induced elevation of the ST segment. 75% of the patients who had post-infarction angina had positive tests. Ventricular premature beats developed in 17% of the subjects during or immediately after the period of exercise. The great majority of patients showed an impaired functional aerobic capacity. In patients with coronary artery disease, the left ventricle has a diminished capacity to augment its stroke volume. In order to increase cardiac output during effort, these patients depend on early elevations of heart rate. A positive exercise stress test, in patients with prior myocardial infarction, appears to be a valuable means of identifying the existence of residual myocardial ischemia resulting from coronary lesions in other vessels. It may be a prognostic tool helping to detect the high risk subjects and it may also be used to precisely determine the effort capacity in individual subjects.