Thirty-four patients were re-examined 12-24 months after mitral valve replacement with the Lillehei-Kaster pivoting disc valve prosthesis. There was an improvement of the kinetics with a significant decrease in arteriovenous oxygen difference compared with oxygen consumption. Cardiac output and stroke volume remained abnormally low, not differing significantly from pre-operative values. Resting pulmonary wedge pressure, pulmonary arterial pressure and pulmonary vascular resistance decreased significantly after operation. Mean values for these parameters remained slightly above normal. Exercise produced a rise in pulmonary wedge pressure, which could be explained partly by a simultaneous deterioration of the left ventricular function, as indicated by high end-diastolic pressures, and partly by a degree of obstruction to the foreward flow at the mitral valve itself. The rise in pulmonary wedge pressures led in turn to increased pulmonary arterial pressure, the latter being aggravated in some patients by a rise in pulmonary vascular resistance. Cardiac output increased during exercise, but usually to a lesser extent than was anticipated from the increase in oxygen consumption. The changes in cardiac output during exercise were due to an increase in heart rate, while stroke volume remained unchanged. The mean diastolic pressure difference across the prosthesis was 8.6 mmHg at rest and 12.7 mmHg during exercise. Subjective improvement and reduction in heart volume, as noted in the present series, corroborated the benificial effect of the operative procedure. The valve were all competent, as judged by cinéangiography, except in a few patients, in whom a small valvular or paravalvular leakage was seen. No thrombo-embolic episodes were recorded during the follow-up period.