In order to investigate the time of ductal closure in the premature infant, we performed multiple echocardiographic examination in each of 42 premature infants. Twenty eight of these infants had spontaneous closure of the ductus arteriosus (median date of closure--3 days). Twelve required medical or surgical closure of the ductus arteriosus and two died of severe lung disease. There was a significant relationship of decreasing birth weight to prolonged patency of the ductus arteriosus.