We investigated the prevalence of overweight and obesity in German schoolchildren and analyzed determinants of overweight. In the context of a randomized intervention study, a baseline cross-sectional assessment was carried out in 2006. During a physical examination, height, weight, skin fold thickness, and upper arm and waist circumferences were measured according to a standardized protocol among 1.079 children aged 6-9 years. Overweight and obesity were classified according to the definitions of the International Obesity Task Force. Parents completed a questionnaire on potential determinants of overweight. Logistic regression models were calculated for determinants of overweight and obesity. The prevalence of overweight was 16.5% in boys and 17.3% in girls and of obesity 3.5% and 3.6%, respectively. Migration (29.4 %) was correlated with overweight and obesity. In particular, among boys with migration background, overweight (24.0%) and obesity (6.6%) were highly prevalent. Higher obesity prevalence was associated with maternal smoking during pregnancy, parental overweight, and low parental education. Indicators for physical inactivity such as watching television more than 1 h per weekday, participation in club sports less than once a week, consumption of sweetened drinks (>or=3 times per week), and skipping breakfast before school were associated with childhood obesity. Our results provide further evidence that parental factors such as migration background and education are strongly associated with body mass of the offspring. Physically inactive children with regular consumption of sweetened drinks and no breakfast were prone to be overweight or obese. Changes of these lifestyle factors as targets of interventions are promising to prevent childhood obesity.