Results of studies of the influence of body mass index (BMI) on the allergic status are controversial. As a part of the Aalst Allergy Study, we assessed the prevalence of the different BMI categories (underweight, normal weight, overweight, and obesity) and a possible association between BMI and atopy in 1576 unselected Belgian schoolchildren, aged from 3.4 to 14.8 yr. BMI was used to determine weight status. Skin prick testing with the most common aeroallergens was performed. A parental questionnaire documented data on respiratory and allergic disorders, demographic characteristics and other potential risk factors for sensitization. Among the total children, 4.1% of the children were underweight, 14.5% were overweight, and 7.4% were obese. More girls than boys were overweight (p = 0.015). In the group of children older than 12 yr, we found more overweight (p = 0.03) and obese (p = 0.004) girls, and more obese boys (p = 0.004) than in the younger age groups. In contrast with reports in the literature, an increased prevalence of allergic sensitization in underweight girls only [adjusted odd ratio (OR(adj)) = 2.9, 95% confidence interval (CI): 1.3-6.4] was documented. A strong association between obesity and exercise-induced respiratory symptoms was found in both boys (OR(adj) = 14.5, 95% CI: 2.9-73.3) and girls (OR(adj) = 4.9, 95% CI: 1.3-17.4). No correlations with allergic respiratory symptoms, eczema, or rhinoconjunctivitis could be documented.