Prophylactic closure of the patent ductus arteriosus has been recommended as a means of decreasing the morbidity of the very low birth weight neonate. This study was undertaken in order to determine potential risk factors involved in the development of the silent ductus, its impact upon both the early cardiorespiratory symptomatology and the subsequent morbidity of the premature neonate, and finally the potential benefit to be derived from prophylactic closure in this presymptomatic stage. Infants with birth weights of 1000 g or less were studied on days 2-3 of life echocardiographically, clinically, and with determination of plasma dilator prostaglandin levels. On entry to the study, those infants with early evidence of silent left-to-right patent ductus arteriosus (PDA) shunting were randomized to receive either prophylactic indomethacin or placebo therapy. Those infants with no evidence of ductal shunting were not treated at all. Infants with silent PDAs had elevated levels of the dilator prostaglandin metabolite 6-keto PGF1 alpha on admission, although they had no echocardiographic abnormalities. No other risk factors for PDA development could be identified. Silent PDA infants had an increased incidence of subsequent symptomatic PDAs, and overall morbidity and mortality when compared with those with no evidence of PDA (silent or symptomatic). Prophylactic ductal closure decreased the incidence of subsequent PDA development, but had no effect on overall morbidity and/or mortality.