Objective: To determine the relation between body mass index and the development of asthma in children.

Design: Prospective study of 4393 asthma-free children followed for up to 14 years.

Setting: Children of participants in the National Longitudinal Survey of Youth.

Methods: Analysis was limited to children who were followed from birth and were asthma-free during the first 24 months of life. The outcome was the development of asthma during follow-up (incident asthma). Body mass index (BMI) was our main predictor of interest. Survival analyses, using time to development of asthma as the main endpoint, were stratified by sex and controlled for race/ethnicity, poverty status, and prenatal maternal smoking.

Results: Asthma developed in 218 (5.0 %) children during the follow-up period. The relation between BMI and incident asthma varied by sex. A BMI 85th percentile at age 2–3 years was a risk factor for subsequent asthma development in boys (hazard ratio (HR) 1.6 95% confidence interval (CI) 1.1, 2.4) but not girls (HR 0.8, 95% CI 0.5, 1.4). Similarly, boys with BMIs always 85th percentile were at increased risk for subsequent asthma development (HR 2.4, 95% CI 1.4, 4.4) but not girls (HR 1.5, 95% CI 0.7, 2.9).

Conclusion: Boys with high body masses may be at an increased risk for developing asthma.