Study aim: To examine the prevalence of obesity and its correlation with blood pressure, waist circumference and other health related risk factors (smoking, alcohol consumption, physical inactivity and TV/PC-screen time) in German youths. Material and methods: A cohort of 831 boys and 808 girls, fifth- to tenth-graders from 3 German high schools (high, middle and low education level) were studied. Anthropometric variables were measured, and declared physical activity, use of audio-visual media, parental socio-economic status etc. were recorded via questionnaire. Endurance performance was examined by 6-min run. Results: In the studied cohort, 6.9% were obese, 10.4% overweight, 73.8% normal weight and 8.9% underweight aged 13.5 [plus or minus] 1.8 years. Obese children had the highest systolic and diastolic blood pressure (p<0.001) and the lowest endurance performance (p less than 0.001). Obese children had a 3-fold and overweight children 2-fold higher risk of hypertension. There were no differences between the obese youths and those of normal weight concerning TV and PC (in min/day) but obese children were less active and had a lower participation rate in organised exercise (p = 0.003 and 0.020, respectively). Obese children smoked more cigarettes per day than their normal weight counterparts (p = 0.014; adjusted for age, gender and school type). No differences were observed concerning alcohol consumption. Obesity, smoking and inactivity showed a clear inverse relation with the parent's school graduation and to the education level of the school the subjects attended. Conclusions: Obesity, but not overweight in children and adolescents, was associated with less physical activity, negative health behaviour and increased blood pressure. The association of risk factors with lower socio-economic status is a task for prevention and a challenge for health care systems in industrialised countries. (Contains 2 tables.)