The UMLS Metathesaurus is a syntactically uniform, concept-based, semantically enhanced representation of many of the world's authoritative biomedical vocabularies. Released several times a year, the Metathesaurus is becoming a common, longitudinally maintained source of the current versions of these vocabularies. As vocabularies become standards for reimbursement, reporting, interoperation, and use by applications, the vocabulary obtained from the Metathesaurus must be consistent with that obtainable from each vocabulary's authority. Effective with the first 2004 release, the Metathesaurus represents new and updated sources "transparently"--both users and applications are able to "see" each vocabulary in the Metathesaurus without any of the small losses of information introduced by abstractions used in previous versions. Thus, the Metathesaurus can continue to provide its many semantic and lexical value-added features while guaranteeing that original sources will be "visible" in intact form. Vocabulary users and application developers will benefit from the enhancements and economies of scale offered by the Metathesaurus, while preserving distinctions between content provided by external authorities and content added as part of the Metathesaurus development and maintenance process.