OBJECTIVE: To determine the prevalence of obesity in asthmatic patients attending at an outpatient clinic, and to investigate its relationships with asthma severity and level of asthma control. METHODS: In a cross-sectional study we recruited patients aged 11 years and older with confirmed asthma diagnosis from the outpatient asthma clinic of Hospital de Clínicas de Porto Alegre, Brazil. They underwent an evaluation by a general questionnaire, an asthma control questionnaire and by pulmonary function tests. Nutritional status was classified by body mass index (BMI). RESULTS: 272 patients were included in the study. Mean age was 51.1 ± 16.5 years and there were 206 (74.9%) female patients. Mean BMI was 27.5 ± 5.3 kg/m², and 96 (35.3%) patients were classified as normal weight, 97 (35.7%) as overweight and 79 (29%) as obesity. There was a significant higher proportion of female than male patients (34.3% vs. 13.2%, p = 0.002) in the obesity group. There were no significant differences with respect to asthma control (p = 0.741) and severity classification (p = 0.506). The FEV1% predicted was significantly higher in the obese than in the non-obese group (73.7% vs. 67.2%, p = 0.037). Logistic regression analysis identified sex (OR = 3.84, p = 0.002) as an independent factor associated with obesity. CONCLUSIONS: This study showed a high prevalence of obesity in asthmatic patients. Obese and non-obese subjects were similar in regard to asthma severity and level of asthma control. Female sex was associated with obesity in this asthma population.