An echocardiographic study of left ventricular diameter was made in 27 patients with aortic regurgitation. Comparison was made with a group of 21 normal subjects and a group of 6 patients with large left ventricles without valvar regurgitation. Total stroke volume, total left ventricular output, and ejection fraction were derived by the method of cubing the left ventricular diameter to give approximate left ventricular volumes at and-diastole and end-systole. The patients were assessed independently and placed into three grades of severity. The eechocardiographic dimensions of patients with mild aortic regurgitation were not significantly different from those of the normal subjects. With increasing severity of aortic regurgitation, there were increases in group values for left ventricular diameter, total stroke volume, and total left ventricular output. The ejection fraction was depressed below the normal range in only one patient with aortic regurgitation. It is concluded that echocardiographic measurement of left ventribular dimensions is of value in assessing the severity of aortic regurgitation.