In 46 preterm infants with RDS the patency of the ductus arteriosus was established by single film aortography or by clinical diagnosis and confirmation at surgery. The estimated left-to-right shunt through the PDA by aortogram correlated well with the heart size and the clinical diagnosis of heart failure. In 14 infants massive cardiomegaly and heart failure with a PDA occurred before the appearance of a heart murmur. Twelve infants had severe RDS and 34 had mild or moderate RDS. Massive cardiomegaly occurred significantly earlier in infants with severe RDS. It is suggested that ductal ligation is indicated when an infant with massive cardiomegaly requires IPPV and whose aortagram shows that all of the contrast material is in the pulmonary arteries and none in the aortic arch. A heart murmur may or may not be present.