BACKGROUND: 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. METHODS: 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. RESULTS: 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). CONCLUSIONS: 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.