Several studies have identified an association between asthma and obesity in women. It remains unclear if this association is due to genuine asthma or to symptoms caused by overweight, at what age the association develops, and whether it is confined to females. To explore the relations between body mass index, asthma, and atopy in a birth cohort of approximately 1,000 individuals. Information on asthma and measurements of lung function, airway responsiveness, and atopy were obtained on multiple occasions between ages 9 and 26. Associations between these outcomes and body mass index were analyzed using generalized mixed linear regression models. Further analyses adjusted for potential covariates including breastfeeding, birth order, parental asthma, and personal and family smoking history. Body mass index was positively associated with asthma, wheeze, asthma treatment, atopy, immunoglobulin E, and inversely with the FEV(1)/FVC ratio in females. There was no significant association with airway responsiveness to methacholine or salbutamol. There was little evidence of an association between body mass index and asthma or atopy in males. Analyses adjusting for potential covariates showed similar findings. Asthma was not associated with a raised body mass index in childhood and childhood asthma did not lead to being overweight as an adult. A raised body mass index is associated with asthma and atopy in women but not men. Population attributable fraction calculations estimate that 28% (95% confidence interval 7-45) of asthma developing in women after age 9 is due to overweight.