To assess the role of dilatation of the mitral valve ring in the genesis of mitral regurgitation in patients with dilated left ventricles, cross-sectional echocardiography (CE) was performed on 23 normal subjects (group 1), 11 patients with congestive cardiomyopathy who had mitral regurgitation (group 2), and 11 patients with congestive cardiomyopathy and no mitral regurgitation (group 3). By performing CE in the long axis, the maximum antero-posterior diameter of MVR in diastole (Dd), the smallest diameter in systole (Ds), and left ventricular end-diastolic dimension (LVED) were measured. Percentage of shortening of MVR in systole (delta D%) and ratio Dd/LVED were calculated. The LVED, Dd, and Ds were significantly higher than normal in Group 2 and Group 3 patients, while delta D% and Dd/LVED were significantly lower. Dd was within normal limits in eight patients in Groups 2 and 3, four of whom had mitral regurgitation. We conclude that dilatation of the mitral annulus occurs only in some patients with dilated cardiomyopathy, and it does not occur in proportion with the degree of dilatation of the left ventricle. Mitral regurgitation, which occurs in association with left ventricular dilatation, may be due to a mechanism independent of mitral ring dilatation, such as loss of sphincteric action of the annulus or malalignment of the papillary muscles.