Among 425 pre-term low birthweight babies who survived more than 48 hours there were 130 with idiopathic respiratory distress syndrome, 73 of whom received ventilation for their disease. The ductus arteriosus was considered patent by clinical criteria in 41 of these babies, all but 1 of whom had a murmur. Of the remaining 32 infants there were 2 babies only who died and both were found to have a patent ductus arteriosus which had not been detected clinically. Another baby died whilst being ventilated for idiopathic respiratory distress syndrome and although autopsy information is not available it seems likely that factors other than a patent ductus arteriosus caused death. The mean age of murmur detection in those ventilated infants considered to have a patent ductus arteriosus was 6 days (range 2-19). Infants without a murmur or other features of a ductus arteriosus did not require to be ventilated beyond day 7 with the exception of the 2 fatalities already mentioned. Thus, relying on clinical criteria and particularly on the presence of a murmur, if sought often, rarely results in missing or seriously delaying the diagnosis of patent ductus arteriosus in babies with severe idiopathic respiratory distress syndrome.