Objective: The aim was to develop a consensus-based method of risk adjustment for in-hospital mortality among children younger than 18 years after surgery for congenital heart disease (designated RACHS-1). Methods: An 11-member national panel of pediatric cardiologists and cardiac surgeons used clinical judgment to place surgical procedures into six risk categories. Categories were refined after review of information from the Pediatric Cardiac Care Consortium and three statewide hospital discharge data sets. The effects of including additional clinical variables were explored by comparing areas under receiver-operator characteristic curves. Results: Among 4602 surgical patients in the Pediatric Cardiac Care Consortium data set and 4493 in the hospital discharge data, 3767 (81.9%) and 3832 (85.3%), respectively, had a single cardiac procedure, and 98.5% and 89.2%, respectively, were able to be assigned to one of six risk categories defined by the panel. Mortality rates showed expected trends (P < .001). For the Pediatric Cardiac Care Consortium data, mortality rates were 0.4% in category 1, 3.8% in 2, 8.5% in 3, 19.4% in 4, and 47.7% in 6; rates were similar in the hospital discharge data. There were too few cases in category 5 to estimate mortality rates. In multivariable models, younger age, prematurity, and the presence of a major noncardiac structural anomaly added to the risk of in-hospital death predicted by risk category alone. Best performance was obtained when cases with multiple procedures were placed in the risk category of the most complex procedure. Conclusion: The RACHS-1 method should adjust for baseline risk differences and allow meaningful comparisons of in-hospital mortality for groups of children undergoing surgery for congenital heart disease.