This echocardiographic study was designed to assess left ventricular function and pulmonary blood flow in ventricular septal defect. Fifty-one patients aged 2 weeks to 21 years were investigated (group 2). Five of 10 operated patients were studied one week after surgical closure of the defect (group 3) and seven 3 to 6 weeks postoperatively (group 4). The control group consisted of 45 normal subjects aged 2 weeks to 21 (group 1). Left atrium/aortic root dimension ratio (LA/Ao) was used to express the pulmonary systemic flow ratio. Left ventricular chamber size was assessed by measurement of left ventricular end-diastolic dimension and volume. The left ventricular mass and the ratio of end-diastolic volume to left ventricular mass were determined to relate the degree of left ventricular hypertrophy to volume overload. Myocardial function was evaluated using ejection phase indices. Ventricular septal thickness, posterior wall thickness, and echocardiographic patterns of septal motion were also studied. The left atrium/aortic root dimension, end-diastolic volume, and left ventricular mass were significantly greater in group 2 patients (P less than 0.05) than in the normal controls. There was a very good correlation between LA/Ao and pulmonary/systemic flow ratio (r = 0.83). Eleven of the group 2 patients showed asymmetric septal hypertrophy (unrelated to shunt size) but left ventricular function as assessed by ejection phase indices appeared unimpaired. Immediately after operation (group 3) there was a deterioration in left ventricular function in 5 patients as shown by reduced ejection fraction, mean velocity of circumferential shortening, and relative changes in minor axis with systole. Septal motion was impaired but returned to normal in 3 to 6 weeks in 7 patients (group 4). Left atrial/aortic root dimension, end-diastolic volume, and left ventricular mass decreased significantly immediately after operation (group 3) but were still raised at 3 to 6 weeks (group 4).