The Society of Thoracic Surgeons (STS) Congenital Heart Surgery Database was queried to document variation of patient characteristics, procedure types, and programmatic case-mix.All index cardiac operations in patients less than 18 years of age in the STS Congenital Heart Surgery Database (July 2016 to June 2020) were eligible for inclusion except patients weighing ≤2.5 kg undergoing isolated patent ductus arteriosus closure. At the hospital level, we describe variations in patient and procedural characteristics known from previous analyses to be associated with outcomes. We also report variations across hospitals of programmatic case-mix.Data were analyzed from 117 sites (90 322 total operations, 87 296 total index cardiac operations eligible for STAT [STS-European Association for Cardio-Thoracic Surgery] 2020 Mortality Score). The median annual total index cardiac operations eligible for STAT 2020 Mortality Score per hospital was 157 (interquartile range [IQR], 94-276). Wide variability was documented in total annual index cardiac operations eligible for STAT 2020 Mortality Score per hospital (ratio 90th/10th percentile = 9.01), operations in neonates weighing <2.5 kg (ratio 90th/10th percentile = 4.09), operations in patients with noncardiac anatomic abnormalities (ratio 90th/10th percentile = 3.46), and operations in patients with preoperative mechanical ventilation (ratio 90th/10th percentile = 3.97). At the hospital level, the median percentage of all index cardiac operations in STAT 2020 Mortality Category 5 was 3.7% (IQR, 1.7%-4.9%), the median percentage of all index cardiac operations in STAT 2020 Mortality Category 4 or 5 was 24.4% (IQR, 19.0%-28.4%), the median hospital-specific mean STAT Mortality Category was 2.39 (IQR, 2.20-2.47), and the median hospital-specific mean STAT Mortality Score was 0.86 (IQR, 0.73-0.91).Substantial variation of patient characteristics, procedure types, and case-mix exists across pediatric and congenital cardiac surgical programs. Knowledge about programmatic case-mix augments data about indirectly standardized programmatic observed-to-expected (O/E) mortality. Indirectly standardized O/E ratios do not provide a complete description of a given pediatric and congenital cardiac surgical program. The indirectly standardized programmatic O/E ratios associated with a given program apply only to its specific case-mix of patients and may represent a quite different case-mix than that of another program.