We have assessed the value of echocardiographic measurements of left ventricular dimensions, as compared to the standard angiographic calculations, in patients (pts) with mitral regurgitation (MR). Of 126 such pts who had undergone echocardiography and cardiac catheterization, 57 were excluded because of associated congenital cardiac malformations, additional significant valvular disease(s) and technically unsatisfactory echocardiographic study. Of the remaining 69 pts, 24 had rheumatic MR (14 with associated mild stenosis); ischemic heart disease was the cause of MR in 1 pts; myxomatous degeneration was found in 12 and congestive cardiomyopathy in 7. The pts were further subdivided in 3 groups according to the severity of MR. Although in all groups echocardiography consistently underestimated left ventricular volumes, there was a statistically significant correlation for end-diastolic volumes, end-systolic volumes and stoke volumes in all three groups. The best correlation was obtained in the calculation of ejection fraction (r = 0.87; P less than 0.001 in mild MR. r = 0,82; P less than 0,001 in moderate MR. r = 0,81; P less than 0,001 in severe MR). The measurement of left ventricular volumes by M-mode echocardiography is generally unsatisfactory in patients with MR. The echo-calculated ejection fraction however, appears to be reliable.