BACKGROUND The association of prevalence of obesity and asthma has increased significantly, this phenomenon seems multifactorial. Obesity is considered a risk factor for asthma. It has been reported positive association between obesity and allergic asthma. In Mexico there is no conclusive data about this issue. METHODS The study was conducted in patients from 4 to 14 years of age, with allergic asthma. A history and physical examination and BMI estimation were performed, as well as the classification of asthma severity. RESULTS We included 244 patients diagnosed with childhood allergic asthma, Fifty two patients (21.31%, OR 3.6) showed some degree of obesity (above the 85 percentile), 15.1% were male and 6% were female. Morbid obesity was found in 40 patients (16.3%). CONCLUSIONS Our study found an increased prevalence of obesity in children with asthma, with a predominance in male gender, and showed a high association between the severity of asthma and increased BMI.