Use of the Kaiser-Permanente Northwest Region data base allowed an examination of the secular trends in the prevalence and incidence of physician-diagnosed asthma and asthma-like disorders in inpatient and outpatient populations over a 20 year period, health care utilization and temporal changes in utilization patterns for asthma, and the natural history of asthma in selected subpopulations. Demographic factors examined were limited to age and sex.