Thirty-one infants with clinical and radiographic evidence of a large left-to-right shunt across a patent ductus arteriosus underwent echocardiographic study to determine the magnitude of shunting. Seven patients were found to have a normal-left atrial dimensions and a normal left atrial/aortic root dimension at the time when physical and roentgenographic findings of a large PDA were most pronounced. In contrast, left ventricular end diastolic dimensions were excessive in these infants. After ductal closure (four, surgery; two, indomethacin, one, spontaneous) the left ventricular end diastolic dimension fell to the normal range. Optimal echocardiographic accuracy in infants with PDA should combine analysis of both atrial and ventricular dimensions; the former, used alone, may be misleading.