The data from a longitudinal study involving 1,445 white boys and girls from the San Francisco Bay area revealed a significant association between the adoption of cigarette smoking in adolescents with a history of either asthma or bronchitis in childhood and in early and middle teenage years. The association, present in both sexes, is stronger for bronchitis than for asthma, with the strength and direction of the association unaffected by such potentially confounding variables as parental smoking, socioeconomic status, psychosocial traits, etc. Age of onset of asthma (primarily in early childhood) preceded the commencement of smoking in 90% of subjects, indicating that cigarette smoking is not causally related to the development of childhood and adolescent asthma. With respect to bronchitis, however, as a result of the data on age of onset and the temporal relationship with the adoption of smoking, the possibility that cigarette smoking among adolescents may contribute to bronchitis cannot be dismissed.