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Background. Schizophrenia is a debilitating chronic mental illness. However, the annual prevalence of schizophrenia is not well understood because of under-representation of schizophrenia patients in epidemiological surveys. This study used multiple administrative claims databases to estimate the annual prevalence of diagnosed schizophrenia in the USA.Method. The annual prevalence of diagnosed schizophrenia in the USA was estimated for different health insurance coverage groups. The prevalence for privately insured individuals was calculated from an administrative claims database of approximately 3 million privately insured beneficiaries covering the period 1999–2003. The prevalence for Medicaid enrollees was calculated from California Medicaid claims covering the period 2000–2002. The prevalence for Medicare and Medicaid/Medicare dual eligibles was estimated using a combination of both databases. Published statistics were used to estimate the prevalence of schizophrenia in the uninsured and veteran populations and to weight the prevalence rates obtained to the population of the USA.Results. The 12-month prevalence of diagnosed schizophrenia in the USA in 2002 was estimated at 5·1 per 1000 lives. The Medicaid population was identified with the highest prevalence rate among the populations studied. Sensitivity analyses taking into consideration the Veterans Affairs population only changed the estimate slightly to 5·3 per 1000 lives.Conclusion. Analyses of administrative claims data contribute to the understanding of the prevalence of diagnosed schizophrenia. [ABSTRACT FROM AUTHOR]