The aim of the present study was to determine the effect of sinus conversion after mitral commissurotomy on the exercise performance of patients with mitral stenosis (MS) and atrial fibrillation (Af). Electric cardioversion was attempted 10 days after successful balloon mitral commissurotomy in 32 patients with MS and Af. Both symptom-limited exercise tests with respiratory gas analysis and constant workload exercise tests with echo-Doppler examinations were performed before, five days and three months after mitral commissurotomy, and five days after successful sinus conversion. The balloon commissurotomy attenuated the increase in transmitral pressure gradient during exercise. However, no significant increase either in peak oxygen uptake (PVO2) or stroke volume were observed even three months after commissurotomy in patients with persistent Af. Sinus conversion was successful in 17 patients and PVO2 increased from 21.4 +/- 4.1 to 23.4 +/- 4.0 ml/min/kg (p < 0.01). The extent of the increase in PVO2 was related to the atrial contribution in transmitral flow (R2 = 0.39, y = 0.81x + 1.2). Sinus rhythm was maintained for three months in 14 of 17 patients. Increased PVO2 was also preserved in these patients. These results suggest that the sinus conversion after mitral commissurotomy has an effect on the exercise performance of patients with MS and Af.