Obesity in children and adolescents is associated with morphologic and functional changes of the vascular wall, suggesting a potential role of juvenile obesity for the development of atherosclerosis later in life. However, no evidence from intervention studies has been provided so far that weight loss in obese children can improve vascular function. Therefore we designed this study including a cohort of obese children before and after a structured weight reduction program in order to answer the question, whether such an intervention can improve vascular function and reverse destructive vascular changes.