In 14 Danish municipalities physicians have gathered weight and height data from 7541 9th grade students (86.7% of all students in 9th grade participating). Overall 25.2% of the population were overweight (body mass index > 90th percentile). Boys were more frequently overweight than girls (29.3% vs. 21.1%) (P < 0.05). 14.1% of the boys and 8.2% of the girls were obese (body mass index > 97th percentile). Categorizing the participating municipalities by socioeconomic status students in municipalities with low status had a significantly higher prevalence of overweight than students in municipalities with high status. Overweight and obesity among Danish school children is a major concern and there are significant social differences in the prevalence of overweight.