AIM: To analyze the side effects of captopril, an angiotensin converting enzyme inhibitor (ACEI) in newborn and young infants. METHODS: Retrospective analysis of side effects in 43 patients with congenital heart disease after cardiac surgery treated with captopril for heart failure during a two-year period. RESULTS: Median age of the patients was 26 days (range 6-310 days), median weight 3.5 kg (range 1.9-7.9 kg). Initial median dose of captopril was 0.17 mg/kg/day (range 0.05-0.55 mg/kg/day), slowly increased over 3-33 days to a maximal median dose of 1.86 mg/kg/day (range 0.2-2.3 mg/kg/day). All patients were additionally treated with diuretics. Side effects occurred in 17 patients (renal impairment or failure in 6, low blood pressure in 8, and oxygen saturation deficit in 3) requiring cessation or interruption in seven patients with renal impairment/failure (n=4), hypotension (n=1) and aorto-pulmonary shunting with low pulmonary perfusion (n=2). The six children who developed renal impairment or failure did so following a median delay of nine days after reaching the final dose and weighed on average 500 g less than the other patients (P=0.046). All side effects were fully reversible. CONCLUSION: Side effects due to captopril were not dose-related in newborns and infants in this study. However, renal side effects occurred more often in smaller infants. Routine monitoring of infants on ACEI should include renal function tests, blood pressure and transcutaneous oxygen saturation measurements.