Serial echocardiography was performed in 51 children with isolated secundum atrial septal defect before and after surgery to measure the effects of chronic right ventricular overload on ventricular function. Right ventricular dilation increased dramatically with growth and with size of the left to right shunt only in the youngest children (body surface area less than 0.5 m2). A lesser effect of growth and no significant effect of shunt size were noted in older children. Although an initial decrease in right ventricular size occurred in the first 3 months after operation, persistent right ventricular dilation remained up to 5 years after closure of the interatrial defect in more than 80 percent of patients. Preoperatively, the ratio of the right ventricular preejection period to ejection time was significantly less than that of normal children. This ratio increased dramatically after operation, exceeding normal values early in the postoperative period in 18 of 48 children and persisting in 6 of 22 after 3 months. Left ventricular dimensions were normal early and late after operation. Left ventricular function was apparently normal, although an exceptionally high shortening fraction was noted in 22 (44 percent) of 51 children after operation. Aortic systolic time interval ratios decreased after operation from high normal to low normal values. It is hypothesized that the persistent enlargement of the right ventricle after operation may be due to the chronic preoperative dilation secondary to chronic interatrial shunting. The abnormally high shortening fraction after operation may result from an abnormal left ventricular geometric configuration or abnormality of filling. It is suggested that surgical closure of the atrial defect in the first 3 years of life may prevent these abnormalities.