The prevalence of pediatric asthma has increased in many parts of the world. This increase started more than 30 years ago and is particularly obvious in studies which document the onset of asthma in native populations when they change to a "Western" lifestyle. Besides a genetic influence, numerous environmental factors have been described for the development of asthma. Genetic factors are unlikely to explain the sharp increase within the short time period and also allergen and pollution exposure or any specific infection does not actually seem to be the main cause for this phenomenon. Another factor, however, that fits well into the geographical and temporal background of the asthma epidemic is the mother's oral contraceptive use. We therefore review the epidemiological association with later asthma in the children, give a summary of estrogen effects on immune function and develop a preliminary theory how oral contraception could influence later pregnancy.