Background: In patients with mitral regurgitation (MR), plasma BNP level was reported to correlate with severity of MR, functional capacity and outcome. However, the determinants of BNP levels and the effects of mitral valve surgery on BNP levels are not clearly elucidated.Method: 38 patients with chronic severe MR undergoing mitral surgery were prospectively enrolled. Patients who had significant renal, pulmonary, coronary or other valvular diseases were excluded.BNP level assay and echocardiographic studies were done before and 6 months after surgery Results: At baseline,enddiastolic(EDVI) and endsystolic left ventricular (LV) volume indexes(ESVI) were 118.6짹25.9, 44.0짹21.3ml/m2, respectively. LV EF was 62.0짹6.0%, and regurgitant fraction(RF) 70.0짹10.2%. Mean preoperative BNP level was 105짹128 pg/mL. In multivariate analysis, preoperative BNP level was independently related with the presence of AF, LA volume index and systolic PAP, LVEF, EDVI, ESVI, and regurgitant fraction were not related. In all, plasma BNP did not decrease after surgery (105짹128 vs. 81짹82 pg/mL, p=NS). Patients who had concomitant MAZE operation for AF(N=14) showed a significant reduction in BNP level from 133짹84 to 67짹47 pg/mL (p=0.02)Conclusion: In patients with chronic severe MR,BNP level was independently related with the presence ofAF, LA size and pulmonary hypertension, but not with LV volume and MR severity. BNP level decreased after successful surgery only in patients who had concomitant MAZE operation for AF