To evaluate the relationship between asthma and overweight in adolescents. Cross-sectional study including 13-14-year-old adolescents in Rio de Janeiro, Brazil. Asthma was defined according to the International Study of Asthma and Allergies in Childhood (ISAAC). Body Mass Index (BMI) was calculated; overweight was defined as BMI > or = 85th percentile. The distribution of asthma and associated symptoms were compared according weight category using the chi(2) test for linear trend. The association between asthma and overweight, controlling for confounding variables, was studied using odds ratios (OR), confidence intervals (95% CI) and logistic regression, stratified by sex. 2,858 (50.1% female) adolescents were studied; 458 (16.1%) were overweight and 13.3% asthmatic. Average BMI was higher among asthmatics (p = 0.01). When stratified, this difference remained only among girls (p = 0.03). Asthma prevalence increased with higher BMI (p = 0.02). Asthma was associated to overweight (OR = 1.51; 95% CI: 1.07-2.13), irrespective of sedentary lifestyle, smoking mother and presence of dog in the domicile, but only among the girls. In Rio de Janeiro adolescents, we found a positive association between asthma and overweight, exclusively among girls. This suggests that health policies related to asthma and overweight/obesity in this age group need to take sex into consideration.