Objectives Atherosclerosis is a disease that begins in childhood; endothelial dysfunction is its earliest detectable manifestation, and primary prevention strategies are likely to be most effective if instituted early. The aim of this study was to characterize the impact of obesity on vascular function in young children and to determine whether an exercise program improves abnormalities in vascular function.Study design The influence of 8 weeks of exercise training was examined in 14 obese subjects, 8.9 +/- 0.4 years of age, with the use of a randomized crossover protocol. Conduit vessel endothelial function was assessed by means of high-resolution ultrasound and flow-mediated dilation of the brachial artery.Results Exercise training did not change subcutaneous fat mass, body weight, or body mass index. FMD in the obese group was significantly impaired relative to matched control subjects at entry (6.00% +/- 0.69% to 12.32% +/- 3.14%, P < .0001). FMD significantly improved with exercise training (7.35% +/- 0.99%, P < .05) in the obese group.Conclusions Conduit vessel FMD, a validated surrogate measure of early atherosclerosis, was impaired in obese children but improved as a result of exercise training. This study supports the value of an exercise program in the treatment of obese children in a primary prevention setting.