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Unified Medical Language System® (UMLS®)


The UMLS includes the Metathesaurus, the Semantic Network, and the SPECIALIST Lexicon and Lexical Tools. The Metathesaurus is the biggest component of the UMLS. It is a large biomedical thesaurus that is organized by concept, or meaning, and it links similar names for the same concept from nearly 200 different vocabularies. The Metathesaurus also identifies useful relationships between concepts and preserves the meanings, concept names, and relationships from each vocabulary.

You can access the Metathesaurus via download, browser, or API.

Need an account? Sign up on the UMLS Terminology Services (UTS) Web site.

Release Information

See Appendix 1 of the UMLS Metathesaurus License Agreement for a list of all vocabularies included in the latest version.

Follow these links for details about the release:


Read more about the Metathesaurus and its data files in UMLS Reference Manual. Relevant chapters include:

More Information

  • MetamorphoSys -- UMLS installation wizard and Metathesaurus customization tool included in each UMLS release.
  • UMLS Database Query Diagrams -- Query diagrams that present use cases and how the data files can be joined (or queried) to find all relevant data for the use cases.
  • Vocabulary FAQ -- Frequently Asked Questions about the sources vocabularies in the Metathesaurus.
  • Vocabulary Mappings -- Current NLM mapping efforts.

Last Reviewed: April 12, 2016