OBJECTIVE: Our goal was to measure the impact of chronic rhinosinusitis (CRS) on the use and cost of health care by adults in a health maintenance organization (HMO). SETTING AND SUBJECTS: In the setting of the Group Health Cooperative, an HMO in Washington State, we conducted a study of all 218,587 adults (≥18 years) who used services during 1994. Using automated data, 20,175 adults were identified with one or more CRS diagnoses during 1994. OUTCOME MEASURES: We identified nonurgent outpatient visits, pharmacy fills, urgent visits, hospital days, and their associated costs (per adult per year). RESULTS: The marginal utilization associated with a diagnosis of CRS was 2.0 nonurgent outpatient visits, 5.1 pharmacy fills, 0.01 urgent visit, and -0.07 hospital day. The marginal total cost of CRS was $206. CONCLUSIONS: Adults with CRS had higher costs primarily because of increased nonurgent outpatient visit and pharmacy fill utilization. The overall direct cost of CRS in the United States in 1994 is estimated to have been $4.3 billion.