The effects of 12 weeks of exercise training without dietary control (n = 30) and exercise training with dietary control by dietition counseling (n = 30) and exercise training with dietary control by dietition counseling (n = 30) on serum cholesterol, serum triglycerides, and total substrate contents were studied in 60 postmyocardial infarction patients. Both groups showed reductions (P less than 0.01) in mean total daily kilocalories consumed (2867 +/- 82 versus 2088 +/- 77 and 2848 +/- 15 versus 1285 +/- 68, respectively); however, no significant change occurred in total body weight. The dietary control group consumed relatively more kilocalories as protein than the group without dietary control (285 of 1,285 versus 389 of 2,088, respectively) and less (P less than 0.05) as fat (443 of 1285 versus 804 of 2,089, respectively). Both groups had lower (P less than 0.01) mean daily dietary cholesterol after 12 weeks (811 +/- 44 versus 232 +/- 17 mg) versus (325 +/- 18 versus 309 +/- 23 mg, respectively). A reduction in serum cholesterol (P less than 0.05) was seen in the dietary control group (270 +/- 8 versus 243 +/- 7 mg/dl) but not in the group without dietary control (260 +/- 6 versus 261 +/- 7 mg/dl). The dietary control group had a lower mean triglyceride level (P less than 0.05) (229 +/- 24 versus 155 +/- 18 mg/dl)) but no differences were seen in the group without dietary control (189 +/- 15 versus 180 +/- 13 mg/dl). It is concluded that significant reductions in caloric intake and daily cholesterol compliment the effects of exercise training in postmyocardial infarction patients by increasing substrate protein:fat consumption ratio and by reducing serum cholesterol and triglycerides. These effects are not seen with exercise training alone.