Background: Childhood obesity and physical inactivity are increasing dramatically worldwide. Both contribute to an increased risk for short-term comorbidities such as diabetes, hypertension, hyperlipidemia, but also to orthopaedic and psychosocial problems, leading to a reduced quality of life. Most importantly, a high body mass index during youth is associated with cardiovascular disease in adulthood. Therefore, the potential health benefits of prevention and reduction in childhood overweight and physical inactivity are of paramount Public Health importance. While interventions to treat childhood obesity have been disappointing, primary prevention is absolutely crucial. KiSS (Kinder- und Jugendsportstudie) is an example of a successful school-based randomized-controlled trial. We implemented a multicomponent physical activity program in first and fifth grade children over a entire school year. Twenty-eight classes were randomly allocated to the intervention (16 classes, n=297) and control (12 classes, n=205) group. The intervention included daily physical education (i.e. two additional lessons per week, on top of three regular lessons), short physical activity breaks during academic lessons, and daily physical activity homework. Compared with controls, children in the intervention group showed statistically significant decreases in the sum of four skinfolds, body mass index (BMI) z-scores, and significant improvements in moderate-and-vigorous physical activity in school, aerobic fitness and a cardiovascular risk score. These findings show that a stringent multi-component school-based physical activity intervention can improve physical fitness and reduce adiposity and cardiovascular risk factors in children. Based on the paucity of longer-term follow-up studies in the literature we propose a three year follow-up to explore whether a one year intervention is sufficient for the positive fitness and health outcomes to persist.Methods: Identical methods like in the initial KISS trial will be used. We will measure the change of our primary (skinfolds, aerobic fitness, physical activity, quality of life) and secondary (cardiovascular risk score) outcomes from the end of the intervention compared to three years later. Expected results: We expect sustained effects on aerobic fitness and body composition. Expected value of the proposed project: Based on the findings of the follow-up study, we will have information if a one year of intervention is sufficient. These results represent a crucial scientific base for a successful Public Health policy transfer and for a wider implementation of KISS.