The aims of this study were to evaluate the structure of the mitral valve (MV) and subvalvar apparatus in patients with rheumatic mitral regurgitation (MR) by echocardiography and to compare the differences in morphologic abnormalities between subgroups of patients with and without mitral valve prolapse (MVP). Two-dimensional and color Doppler echocardiographic examinations were performed in 20 consecutive patients with isolated rheumatic MR and in 15 healthy subjects as controls. Annular diameter, left ventricular end-diastolic dimension, anterior leaflet length, and both leaflet thicknesses were greater in MR than those of controls. Anterior leaflet and chordal lengths were greater in severe MR than in mild or moderate MR. Sixty percent of rheumatic MR patients had nodules on the body or tip of the anterior mitral leaflet and MR was more severe in these patients. Nine of 20 patients (45%) had MVP. MR was more severe in the patients with MVP than those without prolapse. Rheumatic etiology should be suspected in patients with MR when irregular focal thickening of MV, relatively immobile posterior leaflet, eccentric regurgitant jet, and anterior MVP are found in echocardiographic study.