We sought to test whether the mechanisms of mitral regurgitation (MR) evaluated by echocardiography have clinical impact on outcome of patients with significant MR after percutaneous mitral balloon valvuloplasty (PMV). Significant MR was defined as grade ���3/4 with distal MR jet area ���8cm² on color Dopper and the radius of proximal flow convergence region ���0.7cm at the aliasing velocity of 40 cm/s. Three different mechanisms of MR could be defined echocardiographically; 1) leaflet laceration, 2) flail motion with chordae rupture, and 3) commissural MR. Results: A total of 367 patients who underwent PMV between 1995 and 2000 using Inoue balloon were enrolled (272 women; 44짹11 years, mean clinical follow-up 56짹32 months). Significant MR developed in 45 patients (12%) and clinical events occurred much higher comopared to the others (53% vs 12%, p<0.01). Among 45 patients with significant MR, 20 (44%) underwent surgery (group 1), whereas the remaining 25 received medical treatment alone (group 2). Mean mitral gradient (MG) immediately after PMV and MR mechanism are 2 independent variables showing difference between groups after multivariate analysis. In group 2, progressive decrease of MR was noted in 10 and aggravation of MR in 10; commissural MR was more frequently observed in MR decrease group (p=0.03). Conclusions: Significant MR after PMV is relatively infrequent with detrimental impact on clinical course. However, the outcome of patients with MR is variable and treatment strategy seems to be dependent on MR mechanism and degree of hemodynamic improvement after PMV.