OBJECTIVE This study was designed to assess the accuracy of two-dimensional and Doppler echocardiography in determining the mechanism of mitral regurgitation, as compared with direct inspection of the valve at operation. BACKGROUND Valve repair for mitral regurgitation offers substantial advantages over valve replacement, but it is technically more demanding and requires understanding of the mechanism of dysfunction. METHODS We studied 286 patients undergoing mitral valve repair. Intraoperative two-dimensional echocardiography was used to classify mitral leaflet motion as excessive, normal or restricted. Doppler color flow mapping was used to evaluate the direction and origin of the mitral regurgitant jet. Two-dimensional and Doppler echocardiography were compared with intraoperative surgical determination of the mechanism of dysfunction, which also classified leaflet motion as excessive, normal or restricted. RESULTS Two-dimensional and Doppler echocardiography accurately diagnosed the mechanism of mitral regurgitation in 123 (93%) of 132 patients with posterior leaflet prolapse or flail, 30 (94%) of 32 patients with anterior leaflet prolapse or flail, 11 (44%) of 25 patients with bileaflet prolapse or flail, 6 (75%) of 8 patients with papillary muscle elongation or rupture, 31 (91%) of 34 patients with restricted leaflet motion or rheumatic thickening, 21 (72%) of 29 patients with ventricular-annular dilation and 8 (62%) of 13 patients with a leaflet perforation or cleft. Of 13 patients with two mechanisms of dysfunction by surgical inspection, two-dimensional and Doppler echocardiography correctly diagnosed one of the two mechanisms in 12 patients (92%), and both mechanisms in 5 patients (38%). Overall, echocardiographic determination of leaflet motion and Doppler determination of jet direction accurately diagnosed the mechanism of dysfunction in 242 (85%) of 286 patients. CONCLUSIONS Echocardiography before mitral valvuloplasty provides a dynamic appraisal of the mechanism of dysfunction, enabling the surgeon to systematically understand the dysfunction and successfully apply the correct procedures to eliminate mitral regurgitation without valve replacement.