Cardiac surgery patients are urged to resume light physical activity on the first postoperative day, even if cardiac function may not have recovered fully after the operation. To elucidate the postoperative recovery process, we examined cardiac surgery patients with exercise echocardiography before and on the first day after the operation. Patients undergoing on-pump coronary artery bypass grafting were examined with echocardiography during semirecumbent cycle exercise. Patients exercised for five minutes at 10 W intensity and five minutes at 30 W intensity in bed with the upper body supported to approximately 30°. Fourteen patients were studied. Mitral annulus excursion and pulsed wave Doppler from the left ventricular outflow tract indicated postoperatively reduced cardiac stroke volume. Early diastolic tissue velocities of the mitral annulus were reduced, and early trans-mitral flow velocity was increased. The ratio between early mitral flow velocity and early diastolic mitral tissue velocity was increased postoperatively, indicating impaired left ventricular relaxation and increased left atrial pressure. Postoperative systolic mitral annulus tissue velocities were similar to preoperative velocities, indicating maintained systolic function. Postoperative exercise was associated with improvements in myocardial function indices and cardiac stroke volume similar to preoperative improvements. There were no signs of further deterioration in myocardial function during 30 W exercise. In summary, reduced left ventricular diastolic function after surgery resulted in reduced cardiac stroke volume, increased left atrial pressure and a higher rate of perceived exertion on the first postoperative day.