PURPOSE: Oliguria is an occasionally encountering phenomenon after indomethacin administration for the treatment of patent ductus arteriosus. This study was conducted to determine factors contributing to oliguria after indomethacin administration. METHODS: Sixty five infants with patent ductus arteriosus who had been administrated indomethacin in our NICU were enrolled in this study. The patients were divided into two groups; Oliguria group as a study group (n=17), and non-oliguria group as a control group (n=48). The oliguria was defined when urine output is more than or equal to 1.5 mL/kg/hr before indomethacin administration and less than 1.5 mL/kg/hr after indomethacin administration. Patients in oliguria group also included those urine output markedly decreased after indomethacin administration from pre-indomethacin urine output of less than 1.5 mL/kg/hr. Factors including intrauterine age, birth weight, sex, apgar score, respiratory distress syndrome, surfactant instillation, weight loss, ductal size, intraventricular hemorrhage, phototherapy, mechanical ventilation, oxygen therapy, dopamine injection, aminoglycoside injection, serum BUN, creatinine, sodium, potassium, glucose, platelet, hemoglobin, blood pressure were compared between oliguria group with non-oliguria group. RESULTS: Statistically significant factors contributing to oliguria after indomethacin administration included ductal size, gestational age and birth weight (P<0.05). The incidence of oliguria after indomethacin administration increased with increased gestational age, birth weight and ductal size. CONCLUSION: The ductal size, gestational age and birth weight prior to indomethacin administration are important factors contributing to oliguria after indomethacin administration.