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Fact Sheet
Unified Medical Language System®


The National Library of Medicine (NLM) produces the Unified Medical Language System® (UMLS®) to facilitate the development of computer systems that behave as if they "understand" the meaning of the language of biomedicine and health. As part of the UMLS, NLM produces and distributes the UMLS Knowledge Sources (databases) and associated software tools (programs) for use by system developers in building or enhancing electronic information systems that create, process, retrieve, integrate, and/or aggregate biomedical and health data and information, as well as in informatics research.

By design, the UMLS Knowledge Sources are not limited to particular applications; they are multi-purpose. Developers will find that they can be applied in systems that perform a range of functions involving one or more types of information, e.g., patient records, scientific literature, guidelines, public health data. The associated UMLS software tools assist developers in customizing or using the UMLS Knowledge Sources for particular purposes.

UMLS Knowledge Sources and Software Tools

There are three UMLS Knowledge Sources: the Metathesaurus®, the Semantic Network, and the SPECIALIST Lexicon. They are distributed with the Lexical Tools and the MetamorphoSys installation and customization program. NLM updates the UMLS twice a year in May and November.


The Metathesaurus is a customizable, multi-purpose, and multi-lingual vocabulary database that is organized by concept, or meaning. It links alternative names and views of the same concept from different source vocabularies and identifies useful relationships between different concepts. It is built from electronic versions of many thesauri, classifications, code sets, and lists of controlled terms such as:

  • patient care
  • health services billing
  • public health statistics
  • indexing and cataloging biomedical literature
  • basic, clinical, and health services research

All concepts in the Metathesaurus are assigned to at least one semantic type from the Semantic Network, which serves to categorize and disambiguate these concepts for information systems that would normally be unable to do so.

The Metathesaurus contains over 150 source vocabularies, such as MeSH®, RxNorm, SNOMED CT®, ICD-9-CM, ICD-10-CM, CPT®, and LOINC®. Many of the vocabularies are designated to meet US health standards for electronic health records. All vocabularies are available in one of two UMLS common fully-specified database formats, Rich Release Format (RRF) and Original Release Format (ORF). A detailed description of the Metathesaurus appears in the UMLS Reference Manual.

Semantic Network

The Semantic Network consists of:

  1. a set of broad subject categories, or semantic types, that provide a consistent categorization of all concepts represented in the UMLS Metathesaurus, e.g., medical device, clinical drug, and laboratory, and
  2. a set of useful and important relationships, or semantic relations, that exist between semantic types, e.g., treats, diagnoses, and contains.

The scope of the Semantic Network is broad, allowing for the semantic categorization of a wide range of terminology in multiple domains. Major groupings of semantic types include organisms, anatomical structures, biologic function, chemicals, events, and physical objects. The links between the semantic types provide the structure for the Semantic Network and indicate important relationships. A detailed description of the Semantic Network appears in the UMLS Reference Manual.


The SPECIALIST Lexicon provides the word usage information needed for the SPECIALIST Natural Language Processing (NLP) System. The Lexicon is a general English language dictionary that includes many biomedical terms. Coverage includes both commonly occurring English words and biomedical vocabulary. The Lexicon entry for each word or term contains the syntactic, morphological, and orthographic information needed by the SPECIALIST NLP System. The SPECIALIST NLP Tools are a set of Java® programs designed to help users manage lexical variation in biomedical text. The tools use information from the SPECIALIST Lexicon and other data to generate lexical variants of words or terms appropriate for use in indexing and other NLP applications. A detailed description of the SPECIALIST Lexicon and Lexical Tools appears in the UMLS Reference Manual.


MetamorphoSys is the UMLS installation wizard and Metathesaurus customization tool, updated and included in each UMLS release. MetamorphoSys installs the UMLS Knowledge Sources and creates customized Metathesaurus vocabulary subsets. A detailed description of MetamorphoSys appears in the UMLS Reference Manual.

UMLS Terminology Services (UTS)

The UMLS Terminology Services (UTS) is the Web interface for the UMLS Knowledge Sources.  You also will find browsers, downloads, and access to a Web services Application Programming Interface (API) for system developers. Start here to apply for a UMLS license account.  A detailed description of the UTS appears in the UMLS Reference Manual.

Applications and Use Cases of the UMLS

NLM and many other institutions apply the UMLS resources in a wide variety of applications such as information retrieval, natural language processing, creation of patient and research data, and the development of enterprise-wide vocabulary services. NLM uses include MedlinePlus Connect, PubMed,, and the Indexing Initiative. Details about other applications that use the UMLS are available on the UMLS Web site.

Accessing the UMLS Knowledge Sources

NLM distributes the UMLS Knowledge Sources and associated software tools, free of charge to U.S. and international users.

Access to the complete UMLS which includes the Metathesaurus, Semantic Network, and SPECIALIST Lexicon requires an active UMLS Metathesaurus License. Licensees may access the UTS to browse, remotely query, and retrieve data from the Metathesaurus; browse the Semantic Network and SNOMED CT; download the UMLS; and request the UMLS DVD. For instructions on requesting a license and accessing the UTS, see How to License and Access the Unified Medical Language System® (UMLS®) Data. Separate license fees may apply to specific Metathesaurus source vocabularies.

The Semantic Network is available without a license through the UMLS Semantic Network Web site. Distribution is subject to terms and conditions.

The SPECIALIST Lexicon is available without a license through the SPECIALIST NLP Tools Web site. Distribution is subject to terms and conditions.

Do not let the license requirement discourage you from using the Metathesaurus. Setting up the license agreement is quick and easy, requiring only a Web browser, an e-mail address and some basic information about your intended use. Once the license agreement is in place, you may use much of the content of the Metathesaurus under open conditions. Your pre-existing licenses for specific content will cover your use of that content as distributed within the Metathesaurus. Some vocabulary producers who require you to request permission for production uses of their content will generally grant permission free of charge. Read the license carefully to ensure that your intended usage does not incur fees.


See the UMLS Web site for training resources, technical documentation, and source vocabulary documentation.

See the UMLS Reference Manual for detailed information about the purpose, content and file structure of the Knowledge Sources.

Other Fact Sheets in the UMLS series: UMLS MetamorphoSys, UMLS Metathesaurus,, UMLS Semantic Network, SPECIALIST Lexicon, and UMLS Terminology Services.

For general information on NLM services, contact:

National Library of Medicine
Customer Service
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Bethesda, MD 20894
Telephone: 1-888-FINDNLM (1-888-346-3656)
NLM Customer Service Form at

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