Detailed preoperative and postoperative hemodynamic studies were carried out in 12 patients with severe aortic and mitral valve disease in whom replacement of both valves with ball-valve prostheses was carried out. Replacement of both valves resulted in restoration to normal, or nearly normal, the resting values of cardiac output, left ventricular end-diastolic pressure (LVEDP), pulmonary arterial pressure, and pulmonary vascular resistance. However, studies of left ventricular function during muscular exercise revealed abnormalities in the relationships between cardiac output and oxygen consumption in six of seven patients, and in no patient was the left ventricular response to exercise normal when assessed by the relation between LVEDP and stroke volume. It appears likely, therefore, that underlying myocardial disease or coronary arterial disease contributed to the preoperative disability in these patients, and may limit complete symptomatic recovery following operation. The residual impairment of myocardial performance observed in these patients appears to be more pronounced than that previously demonstrated in patients in whom isolated replacement of either the mitral or aortic valve had been performed.