Four of 44 patients who had undergone the Fontan operation had persistent low cardiac output necessitating takedown of the shunt 6 to 65 hours (average 23 hours) postoperatively. All four were in a group of 22 patients with complex lesions other than tricuspid atresia with ventriculoarterial concordance. The development of postoperative right atrial hypertension (average 24 torr), hepatomegaly, marked ascites, and decreasing lung compliance led to severe systemic hypotension with systolic arterial pressure ranging from 55 to 82 torr (average 68 torr), persistent metabolic acidosis, and oliguria despite massive colloid and crystalloid infusions (11,000 ml/m2/24 hr) and inotropic support. At reoperation the atriopulmonary anastomosis, which was found to be wide open, was taken down and an atrial septal defect was created in all patients. Three patients were left with a Glenn shunt and an aortopulmonary shunt to the left lung. One patient had bilateral aortopulmonary shunts. Two patients who survived reoperation had immediate postoperative improvement in systolic arterial and mean right atrial pressure (average 100 torr and 11.5 torr, respectively). Both are well 5 months and 4 years later. Repeat Fontan operation remains a possibility with acceptable risks because of the presence of the Glenn shunt in both patients. We believe that takedown should be considered in patients with persistent low cardiac output after the Fontan operation.