Aim of this study was to evaluate right ventricular performance in patients with mitral stenosis and its modification by balloon valvuloplasty. Right ventricular volumes of 24 patients with postrheumatic mitral stenosis were determined by thermodilution 1 or 2 days before and 1 or 2 days after valvuloplasty. Right ventricular ejection fraction at rest was 43 (36-47)% (median and interquartile range). Right ventricular end-diastolic volume was 100 (86-119) ml/m2. Supine bicycle exercise (50 Watt) reduced right ventricular ejection fraction to 30 (29-37)% (P < 0.0001) and increased right ventricular end-diastolic volume to 124 (112-141) ml/m2 (P < 0.0001). At rest, right ventricular ejection fraction correlated inversely with pulmonary vascular resistance (r = -0.64, P < 0.0001), while no significant correlation with mitral valve area was found. Valvuloplasty increased right ventricular ejection fraction at rest to 48 (44-50)% (P < 0.005), and during exercise to 42 (38-45)% (P < 0.0001). This improvement of right ventricular ejection fraction correlated inversely with the value of this parameter before valvuloplasty (r = -0.88, P < 0.0001) and with the gain in stroke volume (r = 0.57, P < 0.01). The right ventricular function curve, disturbed before commissurotomy, was reestablished by the procedure. In conclusion, at the here investigated stage of mitral stenosis right ventricular function is reversibly impaired. This is predominantly caused by the hemodynamic consequences of the valvular defect and not by an impairment of right ventricular myocardial function.