OBJECTIVES: To evaluate the efficacy of fenoldopam mesylate (dose 0.2 µg/kg/min) in reducing the occurrence of hyperlactataemia (i.e. peak level of blood lactate >2.0 mM/l) during cardiopulmonary bypass (CPB) in paediatric cardiac surgery. Hyperlactataemia occurring during CPB for paediatric cardiac surgery is considered an early biomarker of an increased risk of poor outcome. METHODS: This was a dose/effectiveness clinical study applying Simon’s two-stage optimal design with 5% type I error rate and 90% statistical power. Following parents’ written informed consent, 53 children undergoing elective cardiac surgery with CPB between March 2009 and February 2012 were enrolled. Inclusion criteria were children weighing 3–15 kg scheduled for elective cardiac surgery and with expected CPB time of 60–180 min. Patients requiring surgery with total circulatory arrest were excluded. All patients received fenoldopam infusion at a dose of 0.2 µg/kg/min from the beginning of surgery until the end of CPB. RESULTS: The primary end-point was the evaluation of response to fenoldopam, i.e., blood lactate levels ≤2.0 mM/l. A total of 53 children, median age 5.7 months (range 11 days to 48 months) were enrolled. In the first stage, 18 of 19 (95%) children achieved normalization of lactate values. Then the study was continued to stage II by enrolling an additional 34 patients. At study conclusion, 96.2% of patients showed normalized lactate values. Fenoldopam infusion was well tolerated in all patients. No adverse events were observed. CONCLUSIONS: In this study, fenoldopam at a dose of 0.2 µg/kg/min was well tolerated in paediatric patients undergoing elective cardiac surgery with CPB. In 96.2% of patients, infusion of fenoldopam was associated with intraoperative blood lactate