Our understanding of the natural history of asthma is improving through the establishment of a more precise definition of asthma linked with information from large-scale longitudinal studies. Risk factors for the development of childhood asthma including sex, atopic status, genetic and familial factors, respiratory infections, and outdoor and indoor pollution are now more clearly understood. New information on the relation of viral wheezing episodes in infancy to later childhood asthma is evolving. We now know that children who start wheezing early in life and continue to wheeze at age 6 years are more likely to have a maternal history of asthma, elevated serum IgE levels, and normal lung function in the first year of life. However, at age 6 years they have both elevated serum IgE levels and diminished lung function. Approximately 50% of adults who report having had childhood asthma no longer have symptoms. Airway responsiveness in childhood tends to predict airway responsiveness in adulthood and to be greater in asthmatics with persistent symptoms. Studies of the natural history of asthma support the hypothesis that early therapeutic intervention in mild disease may lead to an improved clinical outcome. Adults exposed to specific occupational environments are at additional risk for the development of asthma. As we learn more about the natural history of asthma, we will have a better understanding of the effect of early diagnosis, environmental control, and therapy on the outcome of the disease.