OBJECTIVE--To determine how severe tricuspid regurgitation influences exercise capacity and functional state in patients who have undergone successful mitral valve replacement for rheumatic mitral valve disease. DESIGN--9 patients in whom clinically significant tricuspid regurgitation developed late after mitral valve replacement were compared with 9 patients with no clinical evidence of tricuspid regurgitation. The two groups were matched for preoperative clinical and haemodynamic variables. Patients were assessed by conventional echocardiography, Doppler echocardiography, and a maximal treadmill exercise test in which expired gas was monitored by mass spectrometry. SETTING--University Hospital of Wales, Cardiff. SUBJECTS--18 patients who had been reviewed regularly since mitral valve replacement. MAIN OUTCOME MEASURE--Objective indices of exercise performance including exercise duration, maximal oxygen consumption, anaerobic threshold, and ventilatory response to exercise. RESULTS--Mitral valve prosthetic function was normal in all patients and estimated pulmonary artery systolic pressure and left ventricular function were similar in the two groups. Right ventricular diameter (median (range) 5.0 (4.3-5.6) v 3.7 (3.0-5.4) cm, p less than 0.01) and the incidence of paradoxical septal motion (9/9 v 3/9, p less than 0.01) were greater in the group with severe tricuspid regurgitation. Exercise performance--assessed by exercise duration (6.3 (5.0-10.7) v 12.7 (7.2-16.0) min, p less than 0.01), maximum oxygen consumption (11.2 (7.3-17.8) v 17.7 (11.8-21.4) ml min-1 kg-1, p less than 0.01), and anaerobic threshold (8.3 (4.6-11.4) v 0.7 (7.3-15.5) ml min-1 kg-1, p less than 0.05)--was significantly reduced in the group with severe tricuspid regurgitation. The ventilatory response to exercise was greater in patients with tricuspid regurgitation (minute ventilation at the same minute carbon dioxide production (41.0 (29.9-59.5) v 33.6 (26.8-39.3) l/min, p less than 0.01). CONCLUSIONS--Clinically significant tricuspid regurgitation may develop late after successful mitral valve replacement and in the absence of residual pulmonary hypertension, prosthetic dysfunction, or significant left ventricular impairment. Patients in whom severe tricuspid regurgitation developed had a considerable reduction in exercise capacity caused by an impaired cardiac output response to exercise and therefore experienced a poor functional outcome. The extent to which this was attributable to the tricuspid regurgitation itself or alternatively to the consequences of right ventricular dysfunction was not clear and requires further investigation.