There is overwhelming evidence, particularly from echocardiography, that the heart of athletes may differ from that of nonathletes matched for age, gender, and body size. Left ventricular mass was larger in most studies in athletes performing predominantly dynamic aerobic and anaerobic sports, in some groups of strength athletes engaged in static training, and in players of ball sports. There is good evidence that the end-diastolic left ventricular internal diameter is enlarged in competitive athletes who perform predominantly dynamic sports, but not in strength-training athletes, which is explained by the different volume load on the heart. There is more uncertainty on left ventricular wall thickening. In sports with high dynamic and low static demands wall thickness is usually proportional to the size of the internal diameter, so that relative wall thickness is not different from control (eccentric hypertrophy). In strength athletes the expected increase in wall thickness without internal diameter change (concentric hypertrophy) was not always observed, possibly related to the variable duration and intensity of the pressure load. In some dynamic sports with high static demands and requiring prolonged training, such as cycling, relative wall thickness may increase. Most studies agree that left ventricular systolic and diastolic function, as measured by various noninvasive techniques, is normal in all types of athletes. The consistency of the results of studies on athletes in the competitive and the resting season, of training of sedentary subjects, and of spinal cord injured patients suggests that variations in physical activity can alter left ventricular structure; genetic factors do not seem to be involved in the size of the left ventricular internal diameter, but have to be taken into account to interpret wall thickness.