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The Centers for Medicare and Medicaid Services (CMS) now make data from the Medicaid and Medicare programs available for research. Prior research has identified important issues in Medicaid data obtained from a commercial vendor.To perform exploratory analyses of Medicaid and Medicare data obtained through CMS.We obtained data from 1999 to 2000 for 5 large Medicaid programs (California, Florida, New York, Ohio, and Pennsylvania), together with the corresponding Medicare data for dual eligibles. We first plotted the number of prescription claims per month. We next plotted the frequency of hospitalization by age group, first using Medicaid data alone, then supplemented with Medicare data. We next plotted the frequency of claims with a diagnosis of complications of pregnancy and childbirth, and of prostate cancers, stratified by sex.The number of prescription claims per month was very stable. Because of cocoverage by Medicare, Medicaid data seemed to miss a substantial number of hospitalizations in those age 45 years and older. Diagnoses of complications of pregnancy and childbirth were uncommon in men and in women older than 60 years, and prostate cancer diagnoses were uncommon in women.In contrast to Medicaid data previously obtained from a commercial vendor, we found no evidence that prescription Medicaid data from CMS were incomplete. We were also reassured by the infrequency of gross diagnostic miscoding. Researchers using Medicaid data to study hospital outcomes should obtain supplemental Medicare data on dual eligibles for studies of persons aged 45 years and older.