Several studies have outlined a possible relationship between an increased body mass index and respiratory allergic diseases, such as asthma and rhinitis.The aim of the study was to analyse the relationship between BMI and lung function, including bronchodilation test, in allergic children. The study included 153 children (103 males, mean age 12.8 years) with allergic rhinitis and mild asthma. All subjects were evaluated performing skin prick test, spirometry, and bronchodilalation test. BMI values were in the normal range as well as lung function. BMI significantly related with FEV1, FVC values and FEV1/FVC ratio both before and after bronchodilation. In conclusion, this study provides the first evidence that BMI is negatively related with bronchial reversibility in children with allergic rhinitis and asthma.As reversibility is related with bronchial inflammation, this finding might underline a link between overweight and allergic inflammation.