Although asthma is on the rise in the United States and elsewhere, data on age-sex-specific patterns of change in various types of health care utilization are scarce. We report on 20-yr trends in the treated prevalence of asthma among members of a large health maintenance organization. Data are presented separately for each of six age-sex categories, and include both the treated prevalence of asthma as well as the treated prevalence of the broader category of chronic airflow obstruction (CAO), defined as asthma, chronic bronchitis, or emphysema. During the period 1967-1987 the treated prevalence of asthma and CAO increased significantly in all age-sex categories except males aged 65 and older. These patterns are in contrast to previous studies of this population that showed that increases in asthma hospitalizations and hospital-based episodes of care were limited primarily to young boys. Not only do these findings support other evidence of a real increase in asthma prevalence, but they also highlight the risks associated with drawing inferences about changing disease epidemiology based on a single type of health care utilization.