This study was designed to assess the effects of exercise on psychosocial functioning in a consecutive series of post myocardial infarction (MI) patients referred for cardiac rehabilitation. Seventy patients with a recent (less than 1 year) MI were randomly assigned to either a program of moderate-high (65–75% VO2 max) or low (<45% VO2 max) intensity exercise training for 12 weeks. In addition to exercise treadmill testing, subjects completed an extensive psychometric test battery including measures of anxiety, depression, stress, and Type A behavior. Although both groups increased their level of cardiorespiratory fitness by 15%, no differences were found in any measures of psychosocial functioning. However, those patients who were initially depressed improved at the completion of the program. The clinical significance of these findings are discussed.