Abstract The hemodynamic data procured in thirtytwo adults with large atrial septal defects are reviewed and compared with those from six cases in children in whom atrial septal defect was complicated by pulmonary stenosis. Elevated right atrial pressure was a common finding in the adults. In the absence of mitral stenosis or constrictive pericarditis, elevation of right atrial pressure in large atrial septal defects may be considered a sign of left ventricular failure. Right ventricular failure in atrial septal defect is characterized by a decrease in the left-to-right shunt, appearance of a right-to-left shunt and a decrease in right ventricular forward output. All this occurs at normal pressures in the common atrium. Biventricular failure, occurring together or in sequence, is a common complication of atrial septal defect in the adult.