The MDS is a core set of items, definitions, and response categories used to assess all of the nation's 1.5 million nursing home residents who reside in facilities participating in the Medicare or Medicaid programs. Further, the Health Care Financing Administration (HCFA) has proposed a rule that would require facilities to computerize MDS data and submit it to state and federal agencies, paving the way for a national database. This article describes the process of testing the reliability of the MDS items in 13 nursing homes in five states. The results demonstrate that MDS data gathered in a research effort attain reliabilities that make such data useful. MDS items met a standard for excellent reliability (i.e., intraclass correlation of .7 or higher) in key areas of functional status, such as cognition, ADLs, continence, and diagnoses. Sixty-three percent of the items achieved reliability coefficients of .6 or higher. Eighty-nine percent of the items in the MDS achieved .4 or higher.