Twenty adult patients with predominant mitral valve disease and variable degrees of tricuspid incompetence (TI) were reinvestigated 6-28 months after mitral valve operation. Postoperatively, 10 of the 20 patients were in sinus rhythm, compared with four of 20 preoperatively. Right ventricula peak systolic pressure decreased from 48 to 33 mm Hg (p less than 0.005), pulmonary vascular resistance declined from 234 to 141 dyn-sec-cm-5 (p less than 0.05), and cardiac index increased from 2.4 to 3.0 l/min/m2 (p less than 0.01) after operation, but right ventricular end-diastolic pressure and right atrial pressure failed to improve. TI, as graded by semiquantitative criteria from right ventricular angiocardiograms taken in the right anterior oblique projection, was decreased unequivocally in only six patients, unchanged in degree in 13 patients, and worse in one. Improvement in TI was associated with an enhancement of systolic shortening of the tricuspid annulus (24 vs 15%, p less than 0.02), whereas in patients with unchanged TI, tricuspid annulus shortening was also unchanged. These data suggest that TI associated with mitral disease is not invariably decreased after mitral surgery, despite improved hemodynamics. A depressed extent of shortening of the tricuspid annulus in systole seems to be important in the pathogenesis of TI.