The metabolic syndrome is associated with insulin resistance, a systemic low-grade inflammatory state, and endothelial dysfunction. These disorders may arise at a very early age in obese children. The aim of this study was to confirm changes in endothelial dysfunction and inflammatory biomarkers in obese prepubertal children and to evaluate the effect of body mass index (BMI) modification on these biomarkers. Biomarkers for inflammation, endothelial dysfunction, and insulin resistance were measured in obese children (47) and healthy controls (47). Baseline pretreatment levels of insulin (P = .019), homeostasis model assessment of insulin resistance (P = .004), soluble intercellular adhesion molecule (sICAM) (P = .003), and C-reactive protein (CRP) (P < .001) were significantly higher in obese children than in controls. After 9 months of treatment, obese children with lowered BMI SD score (SDS-BMI) displayed a significant decrease in insulin (P = .011), homeostasis model assessment of insulin resistance (P = .012), CRP (P = .006), and interleukin-6 (IL-6) (P = .045) levels compared with obese children with stable SDS-BMI; they also displayed a nonsignificant drop in sICAM levels. Similarly, obese children with lowered SDS-BMI displayed a decrease in CRP (P = .005) and IL-6 (P = .065) compared with baseline levels before treatment. In the total obese group, changes in SDS-BMI correlated positively with changes in CRP (P = .035), IL-6 (P = .027), and sICAM-1 (P = .038) levels. Only SDS-BMI was an independent predictive factor for CRP (P = .031), IL-6 (P = .027), and sICAM-1 (P = .033). Prepubertal obese children displayed alterations indicative of endothelial dysfunction, insulin resistance, and inflammatory state. Lowering of the SDS-BMI after 9 months of treatment was associated with an improvement in these variables compared with those in obese children with stable SDS-BMI status.