OBJECTIVE We evaluated the factors related to indomethacin responsiveness of the patent ductus arteriosus (PDA) and subsequent renal and electrolyte abnormalities in a large number of low birth weight infants. METHODS The ductus was evaluated by Doppler echocardiogram or clinical signs after the last administration of indomethacin for 2538 low birth weight infants, through the use of postmarketing surveillance data. RESULTS Multivariate logistic regression analyses demonstrated that clinical closure of PDA was significantly associated with pregnancy-induced hypertension and respiratory distress syndrome. In contrast, a 1-point increase of cardiovascular dysfunction score or a 1-day increase in postnatal age at the first indomethacin treatment decreased the responsiveness of the ductus to indomethacin. Clinical ductal reopening was significantly less likely to occur for each week of increased gestational age. Ductal reopening was more likely for each day of postnatal life at the first administration of indomethacin. Infants with preexisting renal and electrolyte abnormalities and infants whose mothers had received indomethacin tocolysis or who had chorioamnionitis were at increased risk of development of renal impairment. CONCLUSIONS Both antenatal and postnatal factors predict good or poor response to indomethacin therapy for PDA.