OBJECTIVE: To assess how obesity or diabetes mellitus impacts outcomes in patients undergoing cardiac surgery in pediatric hospitals. DESIGN: A multi-institutional, matched case-control study of the Pediatric Health Information System database was performed. SETTING: Tertiary children's hospitals in the United States. PATIENTS: All cardiac surgical cases in patients with obesity or diabetes mellitus between 2004 and 2012 were included. Cases were matched to controls by age, sex, race, and Risk Adjustment for Congenital Heart Surgery score. OUTCOME MEASURES: Mortality, surgical complications, and hospital utilization. Differences in outcome measures were assessed by chi-square and Mann-Whitney tests. P value < .05 was significant. RESULTS: Six hundred twenty-nine cardiac surgical cases (median age 17 years [IQR 12-32]) with obesity or diabetes mellitus were matched to 629 controls. Cases demonstrated lower median household income than those in the control group ($38,031 [IQR $31,900-$48,844] vs. ($41,896 [IQR $32,854-$56,020], P < .001). Mortality was similar between cases and controls (22% vs. 1.9%, P =.692). Surgical complications occurred similarly between cases and controls (13.5% vs. 12.4%, P = .535). Cases had longer intensive care unit length of stay than controls (3 vs. 2 days, P = .001), resulting in longer overall hospital length of stay (5 vs. 4 days, P < .001). Cases also had a higher odds of undergoing mechanical ventilation for >96 hours (OR 2.0, 95% CI 1.1-3.7) and higher rate of total parenteral nutrition use (7.2% vs. 4.5%, P = .040). Median hospital charges were higher in cases (clinical: $6,696 vs. $5,872; laboratory: $14,168 vs. $12,251; pharmacy: $12,971 vs. $10,426; imaging: $6,259 vs. $5,660; P = .030 for all). CONCLUSIONS: The presence of obesity or diabetes mellitus was associated with increased postoperative morbidity, hospital utilization, and cost in patients undergoing cardiac surgery in pediatric hospitals.