The utility of HMO data for the surveillance of chronic diseases.

In an area served by a single medical center that operates as both health maintenance organization (HMO) and fee-for-service clinic, we reviewed existing computerized medical records to determine the prevalence of 11 diseases. Standardized medical care utilization prevalence ratios, comparing the annual prevalences in the two groups, varied from 1.38 for rheumatoid arthritis to 0.60 for liver cirrhosis. Unless supplemented by data from hospitals, physicians, and other sources, HMO data may result in invalid estimates of the prevalence of chronic disease.