BACKGROUND: Although a plethora of literature exists on the impact of body mass index (BMI) in orthopaedic surgery, few have examined its implications in the pediatric cerebral palsy (CP) population. The aim of this study is to evaluate the effect of BMI class on 30-day complications after orthopaedic surgery on children with CP. METHODS: A retrospective analysis of the American College of Surgeons National Surgical Quality Improvement Program (NSQIP) Pediatric participant use files from 2012 to 2013 was conducted. Patients with a diagnosis of CP undergoing any orthopaedic procedure were included and subclassified according to BMI classes: underweight, normal weight, overweight, and obese. Multivariate logistic regressions were performed to evaluate the independent effect of BMI class on total, surgical site, and medical complications as well as unplanned reoperations. RESULTS: A total of 1746 patients were included in our study. These included 345 (19.8%) underweight, 952 (54.5%) normal weight, 209 (12.8%) overweight, and 240 (13.7%) obese children and adolescents. In hip and lower extremity osteotomies, underweight class was an independent risk factor for total complications (P=0.037) and medical complications (P=0.031). Similarly, underweight class was a risk factor for total complications (P=0.022) and medical complications (P=0.019) in spine procedures. Weight class was not independently associated with complications in tendon procedures. Overweight and obesity classes were not associated with any independent increased risk for complications. CONCLUSIONS: With respect to the pediatric CP population, underweight status was deemed an independent predictor of increased complications in osteotomies and spine surgery with no independent increased risk in the overweight or obese cohorts. This information can greatly aid providers with risk stratification, preoperative counseling, and postoperative monitoring as it relates to orthopaedic surgery. LEVEL OF EVIDENCE: Prognostic level III.