Medicare administrative data are fairly accurate in identifying people who affiliate with White or Black racial groups; but less so for other race groups or for Hispanic/Latino origin. Some differences were found between people who were identified as members of these other race groups and those who were missed by the administrative data. Although Medicare administrative files are a useful source of data for analysis of disparities in health care, researchers should be careful to use alternate data sources to test for potential differences between identified and unidentified members of racial and ethnic groups in the attributes being studied.