The objective of this study was to determine the prevalence of overweight and obesity among Brazilian schoolchildren according to gender, age and geographic region. A descriptive association study was conducted on 36,976 schoolchildren (20,914 boys and 16,062 girls aged 10 to 15 years) between 2004 and 2005 using secondary data originating from a school-based cross-sectional epidemiological survey. The cut-off body mass index proposed by the International Obesity Task Force was used as diagnostic criterion for overweight and obesity. The prevalence of overweight and obesity was 11.9% and 2.6%, respectively, with a higher proportion of overweight among girls and of obesity among boys. An association was observed between overweight, age and region, with the demonstration of higher odds ratios for overweight among children aged 10 to 13 years and children from the South, Center-West and Southeast regions. Obesity was significantly associated with gender, age and region, indicating higher odds ratios among boys, children aged 10 to 13 years and children from the South and Southeast regions. Although lower than that observed in most developed countries, the prevalence of overweight and obesity among Brazilian schoolchildren is a matter of concern. In this respect, health policies and strategies should take into account gender and age of the individuals and geographic region of the country.