To assess subclinical inflammation, fasting insulin, and endothelial function before and after exercise in overweight children and adolescents. Twenty-five children (body mass index [BMI] >85th percentile) were assessed for brachial artery flow-mediated dilation (FMD), nitroglycerin-induced dilation, C-reactive protein (CRP), lipids, glucose, insulin, oral glucose tolerance, body composition, aerobic fitness (peak oxygen uptake [VO 2peak]), and blood pressure. Twenty of these persons were equally and randomly assigned to either 8 weeks of stationary cycling or to a non-exercising control group. A baseline correlation was found between CRP and fasting insulin (r = 0.62; P <. 001), which remained significant after adjusting for baseline variables (r = 0.53; P <. 05). After 8 weeks, significant improvements were observed in the exercise group compared with the control group for VO 2peak (exercise group = 21.8 ± 2.1 to 23.2 ± 1.5 mL/kg/minute vs control group = 23.4 ± 1.6 to 20.9 ± 2.2 mL/kg/minute; P <. 05), high-density lipoprotein (HDL) cholesterol (exercise group = 1.02 ± 0.03 to 1.10 ± 0.04 mmol/L vs control group = 1.08 ± 0.07 to 0.99 ± 0.09 mmol/L; P <. 05), and FMD area under the curve (AUC) (exercise group = 746 ± 66 to 919 ± 94 %•sec vs control group = 731 ± 102 to 515 ± 73 %•sec; P <. 05). In overweight children and adolescents, CRP is independently associated with fasting insulin. Eight weeks of aerobic exercise improves fitness, HDL cholesterol, and endothelial function in this group.