Objective: Obesity has been identified as a risk factor for asthma in children. However, in the Netherlands, the obesity prevalence is rising while the asthma prevalence in children is stabilising. The aim of this study is to clarify the association between asthma and Body Mass Index (BMI) in children and whether this association is influenced by sex. Study Design: Parents of 39,316 children (6-16 years) in the south of the Netherlands were invited to complete an online questionnaire on respiratory symptoms, anthropometric variables and several potential confounding factors for asthma and obesity (including sex, birth weight and breastfeeding). Data was analysed by multivariable logistic regression models and an ordinal regression model. Results: The response rate was 24% (n boys=4,743, n girls=4,529). The prevalence of asthma, overweight and obesity was 8%, 15% and 2% respectively. Body mass index -standard deviation Score (BMI-SDS) was related to current asthma (adjusted OR: 1.29; 95% CI: 1.14-1.45, p=0.001). When stratified for sex, asthma and BMI-SDS were only related in girls (Girls: adjusted OR: 1.31; 95% CI: 1.13-1.51, p=0.001. Boys: adjusted OR: 1.01; 95% CI: 0.91-1.14, p=0.72). Conclusions: The positive association between BMI-SDS and asthma is only present in girls, not boys. Future studies into obesity and asthma should correct for sex in their analyses.