The effects of the intensity of exercise training on cardiorespiratory variables were investigated in a consecutive series of men with recent (median 8 weeks) acute myocardial infarction. Forty-five patients were randomly assigned either to a high- (65 to 75% maximum oxygen consumption rate [VO2max]) or to a low-intensity (less than 45% VO2max) exercise group. Patients engaged in medically supervised aerobic training 3 sessions a week for 12 weeks. With training, mean VO2max significantly increased by 11% (2.09 to 2.31 liters/min) within the high group and by 14% (1.93 to 2.21 liters/min) within the low group. Differences between groups were not statistically significant. Both groups also had comparable changes in heart rate, blood pressure and double-product at submaximal and maximal workloads. Analysis of blood lipids revealed that both groups experienced a significant increase in high density lipoprotein cholesterol. There were no significant changes in total serum cholesterol or triglycerides. These findings suggest that within an unselected population of patients after acute myocardial infarction referred for cardiac rehabilitation, low- and high-intensity exercise training produces relatively similar changes in cardiorespiratory variables during the initial 3 months of exercise training.