Rationale: Although asthma is usually considered to originate in childhood, adult-onset disease is being increasingly reported. Objectives:To contrast the proportion and natural history of adultonset versus pediatric-onset asthma in a community-based cohort. We hypothesized that asthma in women is predominantly of adult onset rather than of pediatric onset. Methods: This study used data from the Coronary Artery Risk Development in Young Adults (CARDIA) cohort in the United States over a 25-year period. Adult- and pediatric-onset asthma phenotypes were studied, as defined by age at onset of 18 years or older. Subjects with asthma were categorized by sex, obesity, atopy, smoking, and race by mean age/examination year, using a three-way analysis of covariance model. Natural history of disease was examined using probabilities derived from a Markov chain model. Measurements and Main Results: Asthma of adult onset became the dominant (i.e., exceeded 50%) phenotype in women by age 40 years. The age by which adult-onset asthma became the dominant phenotype was further lowered for obese, nonatopic, eversmoking, or white women. The prevalence trend with increasing time for adult-onset disease was greater among subjects with nonatopic than atopic asthma among both sexes. Furthermore, adult-onset asthma had remarkable sex-related differences in risk factors. In both sexes, the quiescent state for adult-onset asthma was less frequent and also "less stable" over time than for pediatric-onset asthma. Conclusions:Using a large national cohort, this study challenges the dictum that most asthma in adults originates in childhood. Studies of the differences between pediatric- andadult-onset asthmamay provide greater insight into the phenotypic heterogeneity of asthma.