Background: Differences between rheumatic valvulitis and myxomatous degeneration in remodeling of mitral valve (MV) and apparatus in patients with significant mitral regurgitation (MR) have not been demonstrated. Methods: Full-volume transesophageal 3-dimensional images obtained from a total of 75 patients in normal sinus rhythm (11 with normal MV, 12 with rheumatic MR, and 52 with MV prolapse) were analyzed using MVQ (QLab) at mid systole frames. Effective regurgitant orifice area (EROA) was calculated using PISA method. Results: Projectional annulus areas and 3-dimensional annulus perimeters were significantly different among 3 groups, whereas annulus height was not. Lateral to medial and anterior to posterior annulus diameters, and exposed areas of anterior leaflet and posterior leaflet were also different among 3 groups. Spherical index of annulus and the ratio of leaflet area to annulus area were significantly different among 3 groups. By post-hoc analyses, there were significant differences in exposed area of posterior leaflet, spherical index of annulus, and ratio of leaflet area to annulus area between rheumatic and prolapse groups. By multiple linear regression analysis, anterior to posterior annulus diameter (p=0.010) was the only determinant of EROA in patients with rheumatic MR, whereas the ratio of leaflet area to annulus area (p<0.001) and anterior to posterior annulus diameter (p=0.019) were independent determinants of EROA in patients with MV prolapse. Conclusion: Annulus dilation develops in patients with significant MR regardless of valve pathology, but rheumatic annulus is more spherical than myxomatous annulus. The ratio of leaflet area to annulus area is small in rheumatic MV than in myxomatous MV, mainly because of small posterior leaflet size. These results suggest that tailored surgical strategy according to the underlying pathology should be applied in MV repair for patients with MR.