Clinical, radiographic, echocardiographic and operative findings were evaluated in 55 preterm infants with idiopathic respiratory distress syndrome (IRDS) complicated by patent ductus arteriosus (PDA). Pulmonary plethora was detected prior to age seven days in 52 infants, and prior to murmur detection in 42 infants. In those with large shunts, only 35% had cardiomegaly while 78% had a significant increase in cardiothoracic ratio (C/T) on sequential radiographs. Moreover, within 48 hours after ligation, 91% of infants had a significant decrease in C/T. Echocardiographic left atrial to aortic ratio (LA/Ao) was elevated in 71% with large shunts. In one patient with a large shunt there was neither a sequential increase in C/T nor an increased LA/Ao. Severity of left-to-right shunting across a PDA in the newborn was reliably gauged by combined radiographic and echocardiographic evaluation. Either modality alone failed to reflect the presence of a large volume shunt in a number of infants.