Endurance trained athletes have a larger left ventricular end-diastolic diameter and volume and left ventricular mass than sedentary subjects. The left ventricular wall thickness-to-radius ratio is within the normal range in these athletes, suggestive of volume overload hypertrophy. These morphologic changes are the result of the training stimulus rather than genetic influences because a program of intense and vigorous exercise training can induce similar adaptations in sedentary subjects. The training-induced increases in left ventricular end-diastolic diameter and wall thickness occur rapidly. Although these adaptive changes are modest in magnitude with a great deal of overlap between the sedentary and trained subjects, they are useful in maintaining higher levels of cardiac output and stroke volume at maximal exercise in the trained state not only because of cardiac enlargement but because of enhanced left ventricular filling.