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Childhood obesity is an edpidemic in the United States, and is directly associated with risk factors for both cardiovascular disease (CVD) and type 2 diabetes. Endothelial dysfunction (EDF) is thought to be an initiating factor in both these diseases, and recent studies have established the presence of EDF in obese children. While the prevalence of EDF amongst obese children is unknown, EDF in the first two decades of life is predicted to increase both morbidity and mortality from CVD and type 2 diabetes. Purposes. (1) To determine the prevalence of EDF in obese children; (2) To determine whether aerobic exercise was effective in improving endothelial function (EF) in obese children; and (3) To determine whether tissue oxygenation could be used as a clinical marker for EF in obese children. Methods. 49 (24 females) at-risk-for or currently overweight children were evaluated for flow-mediated dilation (FMD) of the brachial artery, tissue oxygentaion, lipids, insulin, glucose, waist and hip circumference and blood pressure. 35 children found to have EDF (17 females), were randomly assigned to an exercise (EX) group, who used Dance Dance Revolution™ (DDR) for a 12-week intervention period, or delayed treatment control (DTC) group. The children in both the EX and DTC groups were tested for all aforementioned measurements as well as for aerobic fitness, markers of nitric oxide production, and inflammatory markers at both baseline and post-intervention. Results. Eighty percent of the 49 children presented with EDF. Children with normal EF (n=9) had lower BMI’s (24.42 ± 3.73 to 29.44 ± 5.11, p=.008), lower diastolic blood pressures (63.78 ± 6.51 to 75.58 mmHg ± 8.77, p=.039) and lower systolic blood pressures (112.00 ± 8.77 to 119.48 ± 9.90 mmHg, p=.000) compared to those who presented with EDF (n=40). Change scores indicated that the EX had a significant improvement in FMD (5.56 ± 5.04% compared to .263 ± 4.54%, p=.008), mean arterial pressure (-5.62 ± 7.03 compared to -1.44 ± 2.16 mmHg, p=.05), weight (2.01 ± 3.38 compared to 5.35 ± 3.97 pounds, p=.017) and relative VO2 (2.38 ± 3.91 compared to -1.23 ± 3.18 mg/kg/min, p=.005) compared to the DTC group. At baseline, FMD was correlated with difference in oxygen saturation normalized by mean blood flow (r=-.342, p=.036) and difference in oxygen saturation normalized by peak blood flow (r=-.517, p=.001). No measurements of oxygen saturation at post-intervention correlated with FMD. Conclusions. While the prevalence of EDF in overweight children is high, 12 weeks of aerobic exercise using DDR not only significantly improved EF, but also aerobic fitness and mean arterial pressure without subsequent improvement in markers of NO production or inflammation. The comprehensive results of this study document the need to further assess the complex relationships between obesity, EF, inflammation and exercise in a larger population of overweight children. Key Terms: Obesity, Children, Endothelial Function, Exercise.