Quality-of-care evaluation must take into account variations in “case mix.” This study reviewed the application of two case-mix complexity-adjustment tools in the Society of Thoracic Surgeons (STS) Congenital Heart Surgery Database: the Aristotle Basic Complexity (ABC) score and the Risk Adjustment in Congenital Heart Surgery (RACHS-1) method. The 2006 STS Congenital Heart Surgery Database Report, the first STS report to incorporate both methods, included 45,635 operations from 47 centers. Each operation was assigned an ABC score in a range from 1.5 (lowest complexity) to 15 (highest complexity), an ABC level in a range from 1 (lowest complexity) to 4 (highest complexity), and a RACHS-1 category in a range from 1 (lowest risk) to 6 (highest risk). The overall discharge mortality was 3.9% (1,222/31,719 eligible cardiac index operations). Of the eligible cardiac index operations, 85.8% (27,202/31,719) were eligible for analysis by the RACHS-1 method, and 94.0% (29,813/31,719) were eligible for analysis by the ABC approach. With both RACHS-1 and ABC, as complexity increases, discharge mortality also increases. The ABC approach allows classification of more operations, whereas the RACHS-1 discriminates better at the higher end of complexity. Complexity stratification is a useful method for analyzing the impact of case mix on pediatric cardiac surgical outcomes. Both the RACHS-1 and ABC methods facilitate complexity stratification in the STS database.