Background: Pleural effusions after bidirectional cavopulmonary anastomosis remain a significant cause of morbidity. Prolonged effusions in such patients have been associated with persistent elevations in plasma renin and angiotensin II. Methods: We conducted a controlled study in 36 patients (median age 8 months) undergoing bidirectional cavopulmonary anastomosis. Enalapril (5 mcg/kg) was administered intravenously within 1 hour of surgery and every 12 hours thereafter in 18 patients; when these patients were tolerating feeds, enalapril was switched to enteral captopril (3 mg/kg/day) every 8 hours. The other 18 patients did not receive perioperative angiotensin converting enzyme inhibitors. Using standardized criteria for discontinuation of chest tubes (< 2 mL/kg/day), volume and duration of pleural drainage were compared between groups. Results. There were no differences between groups in demographic, diagnostic, or hemodynamic factors. There was no difference in cardiopulmonary bypass time between groups and no difference in postoperative pulmonary arterial pressures. The duration of pleural drainage was shorter (2.2 ± 1.4 vs 5.9 ± 1.4 days, p< 0.001) and the volume less during the first 24 hours (4.7 ± 1.2 vs 7.7 ± 2.1 mL/kg, p< 0.001) and overall (10.6 ± 2.4 vs 19.6 ± 4.5 mL/kg, p< 0.001) in patients who received angiotensin converting enzyme inhibitors than those who did not. Readmission for persistent effusions was required in 3 patients who did not receive angiotensin converting enzyme inhibitors and none who did (p= 0.11). Conclusions. Perioperative administration of angiotensin converting enzyme inhibitors is associated with decreased severity and duration of pleural effusions following bidirectional cavopulmonary anastomosis.