Six weeks after acute myocardial infarction, 303 men were randomly divided into exercise and control groups. The exercise group attended the hospital gymnasium twice weekly for a three-month supervised exercise course. Both groups were exercise tested before and after the course and at subsequent follow-up. The exercise group increased their physical fitness greatly compared with the control group. Eight per cent of the exercise group died during the period of follow-up, compared with 14 per cent of the control group; this difference is not significant. There was an apparent improvement in mortality in those with inferior MI who completed the exercise course, which was not seen in those with MI in other sites. For many patients after MI progressive exercise is safe, improves physical fitness and may reduce mortality for those after inferior MI.