Sixty consecutive patients of rheumatic mitral stenosis who underwent percutaneous transvenous mitral commissurotomy were evaluated and followed up for 3 months in order to study the effect of the procedure on left ventricular ejection fraction and to elucidate the pathophysiology of impaired left ventricular function. The response in 16 patients (26.7%) with left ventricular dysfunction (ejection fraction less than 50%) was compared to that in 44 patients with normal left ventricular ejection fraction. Patients with left ventricular dysfunction had relatively larger left ventricular end-diastolic (84 +/- 15 vs. 76 +/- 14 ml) and end-systolic (45 +/- 11.5 vs. 35 +/- 12 ml) volumes. Percutaneous transvenous mitral commissurotomy was successful in all patients. Mitral valve area increased in all patients, from 0.8 +/- 0.2 cm2 to 1.82 +/- 0.37 cm2. After commissurotomy there was a trend towards increase of the left ventricle end-diastolic volumes in both groups of patients. Left ventricular ejection fraction also marginally increased in both groups. A reduced left ventricular compliance due to thickened and fibrotic mitral valve apparatus and excessive afterload due to increased systemic vascular resistance because of low output are possible mechanisms for left ventricular dysfunction in patients with mitral stenosis.