In order to evaluate left ventricular function in atrial septal defect (ASD), 12 cases with ASD, 18 postoperative cases of ASD and 27 normal controls were examined by means of exercise echocardiography using supine bicycle ergometer (50 watts and 3 min). M-mode echocardiograms and left ventricular (LV) short-axis views by two-dimensional echocardiography were subjected to the observation. M-mode echocardiographic studies: ASD group showed an increased stroke dimension (SD) during exercise mainly due to a marked increase of left ventricular end-diastolic dimension ( LVDd ) and a slight decrease of left ventricular end-systolic dimension ( LVDs ). Postoperative ASD group showed an increased SD during exercise mainly due to a marked decrease of LVDs and a slight decrease of LVDd . On the other hand, in normal control group exercise increased SD only by a significant decrease of LVDs . Right ventricular dimension was decreased during exercise in ASD group, but did not change significantly in postoperative ASD and normal control groups. During exercise in ASD group, peak velocity of circumferential fiber shortening (VCF) was increased most markedly, and negative peak VCF was decreased most strikingly. Eight of 12 patients with ASD who showed abnormal interventricular septal motion demonstrated remarkable improvement of the motion during exercise. Two-dimensional echocardiographic studies: Left ventricular short-axis views demonstrated that the shape of the left ventricular cavity changed from flattened to more circular configuration during exercise at end- and early-diastole in ASD group. The shape of the left ventricular cavity at late-systole did not change significantly during exercise in this group. In postoperative ASD group, the left ventricular short-axis view demonstrated circular configuration of the left ventricle at end-diastole both in rest and during exercise. The shape of the left ventricular cavity did not change by exercise. It was concluded that in ASD group cardiac output is increased during exercise by decreasing a left to right shunt at the atrial level and increasing left ventricular filling rate. Therefore, the left ventricular function was found to be good in ASD.