Persistent ductus arteriosus (PDA) complicated the clinical course in 45 (15%) of 299 babies with the respiratory distress syndrome (RDS) presenting over a 3-year period. The incidence of PDA complicating RDS rose with decreasing birthweight and was highest in babies with severe RDS as judged by the use of assisted ventilation. In babies with a birthweight of less than 1500 g, PDA occurred as a complication of RDS in 25% of cases. In babies who recieved intermittent positive pressure ventilation (IPPV) for management of RDS, the incidence of PDA was 35%. Overall mortality for babies with RDS was 17%; when PDA occurred as a complication mortality was 31%. In survivors, spontaneous closure of the PDA occurred in all except one infant. The development of signs of PDA in 19 babies either necessitated a renewal of IPPV or prevented "weaning" from IPPV or from continuous positive airways pressure. 9 (47%) of these babies died, 8 of them after the first week of life. The frequency of PDA complicating RDS and the relatively high mortality in this conservatively-treated group of babies is emphasized. The incidence reported here is likely to increase with the increased survival of very low birthweight infants with severe RDS. The high incidence of late deaths in babies who develop signs of PDA associated with an inability to wean from assisted ventilation suggests that early surgical ligation of the PDA in these babies should be considered. The results of a conservative approach to management of these infants are presented.