Although there have been numerous reports of adverse outcomes for people with asthma who are placed on beta-blockers, there has been no description of how often people with asthma receive prescriptions for beta-blockers. Despite the fact that pharmacy claims are available and can be used for clinical evaluation, there has been no description of a practical surveillance or warning system to recognize and reduce the rate of beta-blocker use in people with asthma. This study used administrative claims data to estimate the prevalence of patients with asthma who also had prescriptions for beta-blockers. Chart audit was used to supplement our understanding of the causes of the problem and its consequences. In the calendar year 1989, in a large midwestern group practice that contracts with a single health maintenance organization (HMO), 3,170 HMO patients presumed to have asthma were identified. Of those 3,170 patients, 44 or 1.4% also had filled prescriptions for beta-blockers. The occurrence of beta-blocker use varied by age group: from less than 1% in patients below 30 years of age, rising to 8.9% in patients aged 60 to 69. Two of the patients with asthma who had prescriptions for beta-blockers were hospitalized for asthma in the study period. In 61% of the cases, different physicians managed the asthma care from those who prescribed the beta-blockers. In the remaining 39%, one physician was responsible for both the asthma care and beta-blocker prescription. We conclude prescribing beta-blockers for individuals with asthma is not uncommon. Current systems of administrative claims data permit the development of warning systems to help avert adverse outcomes.