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Unified Medical Language System® (UMLS®)

About the UMLS

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, and 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.

For additional information on the UMLS:

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 installation 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.

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. 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 must be used to install the Metathesaurus files and is the recommended software tool for customizing the Metathesaurus for specific purposes.

For additional information on the Metathesaurus:


Semantic Network

The purpose of the Semantic Network is to provide a consistent categorization of all concepts represented in the UMLS Metathesaurus and to provide a set of useful relationships between these concepts. All information about specific concepts is found in the Metathesaurus; the Network provides information about the set of basic semantic types, or categories, which may be assigned to these concepts, and it defines the set of relationships that may hold between the semantic types. The current release of the Semantic Network contains 135 semantic types and 54 relationships. The Semantic Network serves as an authority for the semantic types that are assigned to concepts in the Metathesaurus. The Network defines these types, both with textual descriptions and by means of the information inherent in its hierarchies.

The semantic types are the nodes in the Network, and the relationships between them are the links. There are major groupings of semantic types for organisms, anatomical structures, biologic function, chemicals, events, physical objects, and concepts or ideas. The current scope of the UMLS semantic types is quite broad, allowing for the semantic categorization of a wide range of terminology in multiple domains.

For additional information on the Semantic Network:


SPECIALIST Lexicon and Lexical Programs

The SPECIALIST Lexicon has been developed to provide the lexical information needed for the SPECIALIST Natural Language Processing System (NLP). It is intended to be a general English lexicon that includes many biomedical terms. Coverage includes both commonly occurring English words and biomedical vocabulary. The lexicon entry for each word or term records the syntactic, morphological, and orthographic information needed by the SPECIALIST NLP System.

The lexical programs or tools are designed to address the high degree of variability in natural language words and terms. Words often have several inflected forms which would properly be considered instances of the same word. The verb "treat," for example, has three inflectional variants:

Multi-word terms in the Metathesaurus and other controlled vocabularies may have word order variants in addition to their inflectional and alphabetic case variants. The lexical tools allow the user to abstract away from this sort of variation.

For additional information on the SPECIALIST Lexicon and Lexical Programs:



MetamorphoSys is the UMLS installation wizard and customization tool included in each UMLS release. It installs one or more of the UMLS Knowledge Sources; when the Metathesaurus is selected, the user can create customized Metathesaurus subsets. MetamorphoSys may be used to exclude vocabularies that are not required or licensed for use in local applications and to select from a variety of data output options and filters.

For additional information on MetamorphoSys:

Last Reviewed: October 25, 2017