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United States National Library of Medicine National Institutes of Health

Fact Sheet
Unified Medical Language System®


The purpose of NLM's Unified Medical Language System (UMLS®) is to facilitate the development of computer systems that behave as if they "understand" the meaning of the language of biomedicine and health. To that end, 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 multi-purpose. They are not optimized for particular applications, but 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. The lexical tools work more effectively in combination with the UMLS Knowledge Sources, but can also be used independently.

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 flexible lexical tools and the MetamorphoSys install and customization program.

The Metathesaurus is a very large, multi-purpose, and multi-lingual vocabulary database that contains information about biomedical and health-related concepts, their various names, and the relationships among them. It is built from the electronic versions of many different thesauri, classifications, code sets, and lists of controlled terms used in patient care, health services billing, public health statistics, indexing and cataloging biomedical literature, and/or basic, clinical, and health services research. In this documentation, these are referred to as the "source vocabularies" of the Metathesaurus. In the Metathesaurus, all the source vocabularies are available in a single, fully-specified database format.

A complete list of the source vocabularies present in this version of the Metathesaurus appears in Appendix B.4 to this documentation. The list indicates which coding systems and vocabularies are designated as U.S. standards for administrative health transactions in accordance with HIPAA or as target U.S. government-wide clinical standards selected by the Consolidated Health Informatics eGov initiative.

The Metathesaurus is organized by concept or meaning. In essence, its purpose is to link alternative names and views of the same concept together and to identify useful relationships between different concepts. All concepts in the Metathesaurus are assigned to at least one semantic type from the Semantic Network (1.3.2). This provides consistent categorization of all concepts in the Metathesaurus at the relatively general level represented in the Semantic Network. Many of the words and multi-word terms that appear in concept names or strings in the Metathesaurus also appear in the SPECIALIST Lexicon ( The lexical tools ( are used to generate the word, normalized word, and normalized string indexes to the Metathesaurus. MetamorphoSys (1.3.5) must be used to install all the UMLS Knowledge Sources and is the recommended software tool for customizing the Metathesaurus.

The Metathesaurus must be customized to be used effectively.

A complete description of the Metathesaurus and its file structure appears in Section 2 of this documentation.

UMLS Applications

NLM and many other institutions apply the UMLS resources in a wide variety of Applications including information retrieval, natural language processing, creation of patient and research data, and the development of enterprise-wide vocabulary services. NLM's applications include PubMed®, the NLM Gateway,, and the Indexing Initiative. Other examples of UMLS-enabled applications include the National Cancer Institutes Enterprise Vocabulary Services and the Agency for Healthcare Research and Quality's National Guidelines Clearinghouse and National Quality Measures Clearinghouse.

A bibliography of UMLS journal articles from PubMed/MEDLINE covering 1997 to the present in the form of search results may easily be obtained by clicking here. An older bibliography of UMLS articles, including a variety of media, is available by clicking Comprehensive Bibliography 1986-1999.

Obtaining the Knowledge Sources

All UMLS Knowledge Sources and associated software tools are free of charge to U.S. and international users.

The Semantic Network, the SPECIALIST Lexicon, and associated lexical tools are freely accessible on the Internet under open terms, which include appropriate acknowledgment for their use. View the terms and conditions for use of the Semantic Network and of the SPECIALIST Lexicon and Lexical Tools.

To use the Metathesaurus, you must establish a license agreement. This is because the Metathesaurus includes vocabulary content produced by many different copyright holders as well as the substantial content produced by NLM.

Do NOT let the license requirement discourage you from using the Metathesaurus. Setting up the license agreement is quick and easy and is done via the Web. Once the license agreement is in place, much of the content of the Metathesaurus may be used under very open conditions. Your pre-existing licenses for content with use restrictions, e.g., CPT, MedDRA, or NIC, 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.

The complete text of the License Agreement for Use of the UMLS Metathesaurus appears in Appendix A of this documentation.

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Last updated: 23 March 2006
First published: 23 March 2006
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