Objectives: Despite the significant prevalence of elevated blood pressure (BP) and body mass index (BMI) in children, few studies have assessed their combined impact on healthcare costs. This study estimates healthcare costs related to BP and BMI in children and adolescents. Study Design: Prospective dynamic cohort study of 71,617 children aged 3 to 17 years with 208,800 child years of enrollment in integrated health systems in Colorado or Minnesota between January 1, 2007, and December 31, 2011. Methods: Generalized linear models were used to calculate standardized annual estimates of total, inpatient, outpatient, and pharmacy costs, outpatient utilization, and receipt of diagnostic and evaluation tests associated with BP status and BMI status. Results: Total annual costs were significantly lower in children with normal BP ($736, SE = $15) and prehypertension ($945, SE = $10) than children with hypertension ($1972, SE = $74) (P <.001, each comparison), adjusting for BMI. Total annual cost for children below the 85th percentile of BMI ($822, SE = $8) was significantly lower than for children between the 85th and 95th percentiles ($954, SE = $45) and for children at or above the 95th percentile ($937, SE = $13) (P <.001, each), adjusting for HT. Conclusions: This study shows strong associations of prehypertension and hypertension, independent of BMI, with healthcare costs in children. Although BMI status was also statistically significantly associated with costs, the major influence on cost in this large cohort of children and adolescents was BP status. Costs related to elevated BMI may be systematically overestimated in studies that do not adjust for BP status.