Antimicrobial treatment patterns, resource utilization, and charges associated with acute sinusitis in patients with asthma were studied. Asthma patients with at least one claim for acute sinusitis were identified by International Classification of Diseases, 9th Revision code in data for 1992-1994 from a large health care insurer in New England. A sinusitis episode was defined on the basis of the pattern of antimicrobial prescribing. Antimicrobials used to treat sinusitis, rates of resource utilization, and overall charges for therapy were determined. A total of 2,633 sinusitis episodes were identified in the records of 34,348 asthma patients. The most frequently prescribed antimicrobial for initial treatment of acute sinusitis was amoxicillin trihydrate (32.2% of patients). Initial therapy was successful in 2,199 episodes (83.5%), initial therapy failed in 250 episodes (9.5%), and relapse occurred in 184 episodes (7.0%). Successfully treated patients had fewer outpatient visits on average (1.42 per episode) and fewer laboratory and diagnostic tests (0.046) than did patients in whom initial therapy failed (1.76 and 0.056 per episode, respectively) or patients who relapsed (1.68 and 0.049). The mean total charge for sinusitis care was $147.61 per episode for successfully treated patients, $242.95 per episode for patients unresponsive to treatment, and $205.49 per episode for patients who relapsed. Antimicrobial treatment of acute sinusitis varied widely in patients with asthma. Resource utilization was associated with the success of treatment.