Abstract Background Single-ventricle palliation (SVP) for children with unbalanced atrioventricular septal defect (uAVSD) is thought to carry a poor prognosis, but limited data have been reported. Methods We performed a retrospective review of children with uAVSD who underwent SVP at a single institution. Data were obtained from medical records and correspondence with general practitioners and cardiologists. Results Between 1976 and 2016, a total of 139 patients underwent SVP for uAVSD. A neonatal palliative procedure was performed in 83.5% of these patients (116 of 139), and early mortality occurred in 11.2% (13 of 116). Ninety-four patients underwent stage II palliation, with an early mortality of 6.4% (6 of 94). Eighty patients (57.6%) underwent Fontan completion, with an early mortality of 3.8% (3 of 80). Interstage mortality was 11.7% (12 of 103) between stages I and II and 17.0% (15 of 88) between stage II and Fontan. Long-term survival was 66.5% (95% confidence interval CI, 57.9%-73.9%) at 5 years, 64.4% (95% CI, 55.5%-72.0%) at 15 years, and 57.8% (95% CI, 47.5%-66.8%) at 25 years. Survival post-Fontan was 94.9% (95% CI, 86.9%-98.0%) at 5 years, 92.0% (95% CI, 80.6%-96.8%) at 15 years, and 82.4% (95% CI, 61.5%-92.6%) at 25 years. Risk factors associated with death or transplantation were aortic atresia (hazard ratio HR, 5.3; P  = .03) and hypoplastic aortic arch (HR, 2.5; P  = .02). Atrioventricular valve operations were required in 31.7% of the patients (44 of 139), with 31.8% of them (14 of 44) requiring a further operation. Conclusions Children undergoing SVP for uAVSD have substantial mortality, with <60% survival at 25 years. However, survival of children who achieve Fontan completion is better than has been reported previously.