Radionuclide cineangiography at rest and during exercise was utilized to assess left ventricular function in eight patients with atrial septal defect (ASD). All patients had normal left ventricular ejection fraction (LVEF) at rest, but during exercise, four patients had subnormal increments in LVAF and two other patients manifest a decrease in LVEF below 55%, the lower limit of normal. These two patients had exertional dyspnea and orthopnea and also had significantly higher pulmonary to systemic flow ratios than the other 6. After operation, all patients are asymptomatic and all have normal LVEF both at rest and during exercise. Thus, left ventricular dysfunction in patients with ASD is due in part to reversible mechanical factors rather than to intrinsic, irreversible myocardia dysfunction.