Introduction The aim of the study was to determine the prevalence of overweight among under-5-year-old children in Brazil and investigate its associations with sociodemographic characteristics, exclusive breastfeeding, number of siblings and birth weight.

Methods Cross sectional population based study, conducted in the five geopolitical regions of Brazil, with a sample of 6397 children. The nutritional classification was done using the 2006 WHO growth curves. Were considered overweight the children with a z-score higher than two SDs above the weight for height median.

Results The prevalence of overweight among under-5-year-old children in Brazil was 12%. The outcome was 22% higher in males (RP=1.22; 95% CI 1.02 to 1.47; p=0.030). There was a linear inverse association: the younger the child, the higher the prevalence of overweight (p=0.032). The white children had a prevalence of overweight 22% higher than the non-white ones. The higher the birth weight, the higher the prevalences of overweight (p=0.000). Children who were breastfed up to 120 days had a prevalence of overweight 34% higher compared to the ones who were breastfed for more than 120 days.

Conclusion The prevalence of obesity was higher in males, in under-1-year-old, white, with a birth weight of <3500 g, exclusively breastfed up to 120 days children.