To assess the role of mitral valve prolapse (MVP) in the pathogenesis of mitral regurgitation (MR) in rheumatic mitral valve disease (RMD), we performed phonocardiography (PCG), transthoracic and transesophageal two-dimensional and color Doppler (CD) echocardiography in 22 patients with RMD including three with pure mitral stenosis (MS), 11 with predominant MS, six with predominant MR and two with pure MR. Results were as follows: 1. Prolapse of the mitral valve (MV) was differentiated from systolic ballooning of the whole MV by the findings that the anterior leaflet's tip (rough zone) protruded into the left atrium with an acute angle between the body (clear zone) and rough zones of the anterior MV and that the posterior leaflet protruded markedly above the level of the mitral ring. 2. MR was detected in six patients (slight MR) by only the CD method and in 13 (mild, moderate or greater MR) by both the PCG and CD methods. 3. MR was absent or slight in five patients (three of pure MS and two of predominant MS) without valve thickening and with systolic ballooning of the whole valve due to commissural fusion. 4. Mitral valve abnormalities related to significant (mild, moderate or severe) MR were dependent on valve thickening (five patients), prolapse of the leaflet's tip toward the left atrium (four), or both (four). 5. An apical systolic click was found in only one of the nine patients with systolic ballooning, but in four of 11 with MVP. 6. The MR murmur in six of the nine patients with valve thickening showed the decrescendo or flat contour, but that in four of the eight patients with MVP showed a crescendo contour. From these results, we concluded that mitral valve prolapse should be considered as one of the important causes of mitral regurgitation in rheumatic mitral valve disease.