To determine the incidence of major cardiovascular complications in outpatient cardiac rehabilitation programs, we obtained data from 167 randomly selected cardiac rehabilitation programs via mailed questionnaires and follow-up telephone calls. These 167 programs reported that 51 303 patients exercised 2 351 916 hours from January 1980 through December 1984. Twenty-one cardiac arrests (18 in which the patient was successfully resuscitated and three fatal) and eight nonfatal myocardial infarctions were reported. The incidence rates per million patient hours of exercise were 8.9 for cardiac arrests (one per 111 996 patient-hours), 3.4 for myocardial infarctions (one per 293 990 patient-hours), and 1.3 for fatalities (one per 783 972 patient-hours). There was no statistically significant difference in frequency of these events among programs of varying size or extent of electrocardiographic monitoring. These data indicate that current cardiac rehabilitation practice allows for prescribed supervised exercise by patients with cardiovascular disease to be performed at a low risk of major cardiovascular complications.