The development of late tricuspid regurgitation following mitral valve replacement is accompanied by a severe reduction in exercise capacity and a poor functional outcome. In this study, we compared the clinical and echocardiographic characteristics of two matched groups with (n = 13) and without (n = 13) clinically significant tricuspid regurgitation. The preoperative pulmonary artery pressures and symptom durations were similar, but tricuspid regurgitation at palpation was detected only in patients who later developed severe tricuspid regurgitation (5/13 vs. 0/13; p < 0.02). None of the patients had echocardiographic evidence of rheumatic tricuspid valve disease at the time of the study, but the tricuspid annular diameter (3.7 +/- 0.5 cm vs. 3.2 +/- 0.4 cm; p < 0.05) and right ventricular diameter (4.9 +/- 0.4 cm vs. 4.0 +/- 0.8 cm; p < 0.01) were greater in patients who had developed severe late tricuspid regurgitation. Echocardiographic parameters of left ventricular function and Doppler estimated pulmonary artery systolic pressures were similar in the two groups, and no evidence of prosthetic dysfunction or aortic valve disease was found. These results imply that late tricuspid regurgitation following mitral valve replacement develops as a result of dilation of the tricuspid annulus associated with right ventricular decompensation. The persistence of uncorrected tricuspid incompetence would seem to be an important contributory factor, and its accurate detection and correction at the time of initial surgery may prove to be the most effective means of preventing the development of this important complication of mitral valve replacement.