Obesity has been pointed out as a risk factor for higher prevalence of asthma and asthma-related symptoms in adolescents. The objective was to evaluate the relationship between the prevalence of asthma and obesity in adolescents living in Santa Maria and surroundings (state of Rio Grande do Sul, southern Brazil), applying the International Study of Asthma and Allergies in Childhood (ISAAC) protocol. A total of 4,010 of 6,123 schoolchildren, 13 to 14 years of age, enrolled in the ISAAC phase III protocol (asthma core questionnaire) and were nutritionally evaluated: height, weight, and triceps skinfold (TSF) measurements. Prevalence of asthma (wheeze in the last 12 months) and prevalence of severe asthma (two or more affirmative responses to: more than 4 acute attacks of asthma, speech disturbance, sleep disturbance, wheezing with exercise) were evaluated and compared according to their nutritional status: obese and non-obese. Obese adolescents were defined by body mass index (BMI, in kg/m(2)) > or =85th percentile and TSF > or =85th percentile. Obese and non-obese groups were compared for prevalence of asthma and asthma severity using the Chi-square test and odds ratio (OR) with 95% confidence interval. Analyzing all adolescents, we observed a significant positive relationship between the prevalence of obesity and affirmative responses to "wheeze ever" (OR = 1.28; 95% CI 1.08-1.52), "wheezing with exercise" (OR = 1.36; 95% CI 1.11-1.66), "asthma ever" (OR = 1.29; 95% CI 1.03-1.62), and severe asthma (OR = 1.55; 95% CI 1.12-2.14). Among the boys, there was a significant positive association between obesity and "wheeze ever" (OR = 1.49; 95% CI 1.13-1.86). In girls, there was a significant positive relationship with "asthma ever" (OR = 1.38; 95% CI 1.01-1.88) and "wheezing with exercise" (OR = 1.36; 95% CI 1.11-1.66). This cross-sectional study with adolescents living in the southern region of Brazil showed that there is a positive association between obesity and prevalence of asthma symptoms and asthma severity, a finding mainly confined to girls.