As part of a prospective national survey on morbidity and mortality of all infants born at less than 32 weeks and/or weighing less than 1500 g in the Netherlands we studied the incidence of symptomatic patent ductus arteriosus (PDA) in infants surviving at least 24 hours. In 1252 of the 1338 infants the presence or absence of PDA could be established. The incidence of PDA was 10.7%. Logistic regression analyses revealed that idiopathic respiratory distress syndrome (IRDS) was the most predictive factor, followed in order of importance by septicaemia, birthweight, prolonged rupture of membranes and gestational age. None of the other maternal and perinatal factors showed any association with the occurrence of PDA. Furthermore, bronchopulmonary dysplasia (BPD) and necrotising enterocolitis (NEC) were found to be sequelae of PDA. The combination of IRDS, very pre-term birth and septicaemia should be considered as high risk for the development of PDA.