Physical activity performed by patients with coronary heart disease is a two-edged sword. A number of biological changes produced by regular exercise may reduce the risk of future cardiac events, while the increase in cardiac work produced by this same exercise can predispose the patient to sudden cardiac death. Data from observational studies as well as randomized clinical trials demonstrate a lower cardiac mortality rate for men participating in exercise rehabilitation programs vs nonparticipants. Overall, exercise program participants appear to experience a reduction of approximately 25% in cardiac and all-cause mortality, but no single study has provided definitive results. During medically supervised exercise, the risk of cardiac death based on reports of programs in the United States is approximately one event in every 60,000 participant-hours of exercise. At this rate, a typical rehabilitation program that has 95 patients exercising 3 h.wk-1 could expect a sudden cardiac death during an exercise session once every 4 yr. No data have been published on the morbidity or mortality benefits or risks of home-based exercise or for women participants. Also, the contribution of continuous electrocardiographic monitoring to the safety of exercise training of cardiac patients is yet to be defined.