Background: Exercise intolerance is an important prognostic factor, and thus a common indication for surgery in patients with mitral regurgitation (MR). However, determinants of exercise capacity have not been well described. Furthermore, impact of mitral surgery on exercise capacity is not clear. The purpose of this study was to test whether mitral surgery improves exercise capacity and to identify the determinants of exercise capacity in MR patients. Methods: Eighteen patients (mean age: 5112 years, M:F=8:10) with severe MR undergoing mitral surgery were prospectively enrolled in the study. Patients with mitral stenosis or other valve diseases more than mild degree were excluded. B type-natriuretic peptide (BNP) level, cardiopulmonary function test and echocardiograms were obtained at baseline and 6 months after surgery. Results: At baseline, mean end-diastolic and end-systolic left ventricular (LV) volume and ejection fraction were 18250, 6319ml and 654%, respectively. Regurgitant orifice area were 0.90.4cm2. BNP level and maximum oxygen consumption (VO2max) were 11086 pg/ml and 19.46.6 ml/kg/min, respectively. Preoperative VO2max was significantly related with early diastolic mitral annulus velocity (E���) (r=0.59, p<0.05), and early diastolic transmitral flow velocity/E��� ratio (E/E���) (r=-0.75, p<0.05). However, LV ejection fraction, MR severity (regurgitant orifice area), age and BNP level did not show significant relationship with VO2max. BNP (11086 vs 7954 pg/ml, p=NS) and VO2max (19.46.6 vs 20.96.9 ml/kg/min, p=NS) did not change after surgery. Conclusion: Preoperative exercise capacity was closely related with diastolic parameters and successful elimination of MR by corrective surgery did not improve exercise capacity in patients with severe MR. These findings underscore the importance of LV diastolic function as a probable cause of exercise intolerance in these patients.