From 1985 to 1995, the Aragón School Health Examination Surveys staff has surveyed the whole population of children (age 6-7 years) and adolescents (age 13-14 years). A total of 90,997 children (age 6-7 years) were examined in the nine cross-sectional surveys conducted between 1985 and 1995. A total of 106,284 adolescents (age 13-14 years) were also examined. We defined overweight when the body mass index was > or = 95th percentile. The measure of association between overweight and the other variables studied was the odds ratio, which was calculated by logistic regression. In the children and adolescents studied, the probability to be overweight was higher in the rural than in the urban areas. In the adolescents, the probability to be overweight was higher in the public than in the private schools. Our results also show a significant increase in the prevalence of overweight children in the region of Aragón (Spain) during the 1985-1995 decade, especially in males. Prevention of overweight in the populations at risk would help reduce social inequality in health, a major challenge for public health policy.