To determine the preoperative and postoperative effect of atrial fibrillation (AF) on exercise capacity in mitral stenosis, 12 digitalized patients in AF (7 women and 5 men, age 52 +/- 6.1 years) and 10 in sinus rhythm (5 women and 5 men, age 46 +/- 5 years) underwent maximal cardiopulmonary exercise testing according to Weber's protocol and Doppler echocardiographic examination before and at 3 and 6 months after mitral valve replacement. The ratio of right ventricular acceleration to ejection time was used as an estimate of mean pulmonary artery pressure. Preoperative exercise duration (6.8 +/- 1 vs 8 +/- 2 minutes), peak oxygen consumption (9.7 +/- 3 vs 12.3 +/- 3 ml/kg/min), and right ventricular acceleration to ejection time ratio (0.34 +/- 0.07 vs 0.34 +/- 0.08) were not significantly different between patients with AF and those in sinus rhythm. Postoperative improvement in these parameters was lower in patients with AF than in those in sinus rhythm: exercise duration at 3 months, 7.5 +/- 2 vs 11.9 +/- 2 minutes (p < 0.001); at 6 months, 9 +/- 2 vs 12 +/- 2 minutes (p < 0.001); peak oxygen consumption at 3 months, 10.8 +/- 3 vs 17.5 +/- 3 ml/kg/min (p < 0.001); and at 6 months, 11.9 +/- 3 vs 17.8 +/- 3 ml/kg/min (p < 0.001); right ventricular acceleration to ejection time ratio at 3 months, 0.35 +/- 0.08 vs 0.42 +/- 0.05 (p < 0.05); and at 6 months, 0.38 +/- 0.05 vs 0.44 +/- 0.05 (p < 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)