OBJECTIVES: The present study was designed to investigate the dimensions of mitral valve annulus in the presence of mitral regurgitation. METHOD: Fifty-four patients were examined. On transthoracic echocardiographic images, we performed linear measurements in the parasternal plane in order to define the size of the left ventricle, left atrium, and mitral valve annulus. We compared these findings with those obtained in 16 control subjects. RESULTS: Twenty-one patients with mild or moderate mitral regurgitation demonstrated no significant change of the mitral valve annulus compared with the control group (P > 0.05). Seventeen patients with severe mitral regurgitation (grade of 4) had a significant increase of the dimensions of the mitral valve annulus, left ventricle, and left atrium (P < 0.05). The etiology of mitral regurgitation was degenerative in 32 patients, rheumatic in 2 patients, and mitral valve prolapse in 4 patients. All patients had normal left ventricular systolic function. Thirty-one patients were in normal sinus rhythm, and seven were in atrial fibrillation. CONCLUSIONS: The measurement of the diameter of the mitral valve annulus is feasible with transthoracic echocardiography. In addition to the evaluation of mitral valve leaflets and subvalvular apparatus, the measurement of the mitral valve annulus is important in the evaluation of mitral regurgitation, as its enlargement is indicative for severe mitral regurgitation.