The impact of age and weight on outcomes after the Fontan operation is unclear. Previous analyses have suggested that lower weight-for-age z-score is an important predictor of poor outcome in patients undergoing bidirectional Glenn. We evaluated variation in age, weight, and weight-for-age z-score at Fontan across institutions, and the impact of these variables on postoperative morbidity and mortality.Patients in The Society of Thoracic Surgeons Congenital Heart Surgery Database undergoing the Fontan operation (2000 to 2009) were included. Center variation in age, weight, and weight-for-age z-score were described. Multivariable analysis was performed to evaluate the impact of age, weight, and weight-for-age z-score on in-hospital mortality, Fontan failure (combined in-hospital mortality and Fontan takedown/revision), postoperative length of stay, and complications, adjusting for other patient and center factors.A total of 2,747 patients (68 centers) were included: 61% male; 45% right dominant lesions (38% left dominant, 17% undifferentiated). An extracardiac conduit Fontan (versus lateral tunnel) was performed in 63%; 65% were fenestrated. Median age, median weight at Fontan operation, and proportion with weight-for-age z-score less than -2 varied across centers and ranged from 1.7 to 4.8 years, 10.5 to 16.1 kg, and 0% to 30%, respectively. In multivariable analysis, age and weight were not significantly associated with outcome. Weight-for-age z-score less than -2 was associated with increased in-hospital mortality (odds ratio 2.73, 95% confidence interval: 1.09 to 6.86), Fontan failure (odds ratio 2.59, 95% confidence interval: 1.24 to 5.40), and longer length of stay (+1.2 days, 95% confidence interval: 0.1 to 2.4).Weight-for-age z-score less than -2 is associated with significant morbidity and mortality after the Fontan operation independent of other patient and center characteristics.