Rationale: Asthma and obesity often occur together in children. It is unknownwhether asthma contributes to the childhood obesity epidemic. Objectives: We aimed to investigate the effects of asthma and asthma medication use on the development of childhood obesity. Methods: The primary analysis was conducted among 2,171 nonobese children who were 5-8 years of age at study enrollment in the Southern California Children's Health Study (CHS) and were followed for up to 10 years. A replication analysis was performed in an independent sample of 2,684 CHS children followed from a mean age of 9.7 to 17.8 years. Measurements and Main Results: Height and weight were measured annually to classify children into normal, overweight, and obese categories. Asthma status was ascertained by parent-or self-reported physician-diagnosed asthma. Cox proportional hazards models were fitted to assess associations of asthma history with obesity incidence during follow-up. We found that children with a diagnosis of asthma at cohort entry were at 51% increased risk of developing obesity during childhood and adolescence compared with children without asthma at baseline (hazard ratio, 1.51; 95% confidence interval, 1.08-2.10) after adjusting for confounders. Use of asthma rescue medications at cohort entry reduced the risk of developing obesity (hazard ratio, 0.57; 95% confidence interval, 0.33-0.96). In addition, the significant association between a history of asthma and an increased risk of developing obesity was replicated in an independent CHS sample. Conclusions: Children with asthma may be at higher risk of obesity. Asthma rescue medication use appeared to reduce obesity risk independent of physical activity.