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United States National Library of Medicine National Institutes of Health

Fact Sheet
UMLS® 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®, and (2) a set of useful and important relationships, or Semantic Relations, that exist between Semantic Types. This section of the documentation provides an overview of the Semantic Network, and describes the files of the Semantic Network. Sample records illustrate structure and content of these files.

Structure and Content of the UMLS Semantic Network

The scope of the UMLS 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, physical objects, and concepts or ideas. The links between the semantic types provide the structure for the network and represent important relationships in the biomedical domain. The primary link between the semantic types is the 'isa' link. The 'isa' link establishes the hierarchy of types within the Network and is used for deciding on the most specific semantic type available for assignment to a Metathesaurus concept. There is also a set of non-hierarchical relationships, which are grouped into five major categories: `physically related to,' `spatially related to,' `temporally related to,' `functionally related to,' and `conceptually related to.'

The information associated with each semantic type includes a unique identifier, a tree number indicating its position in an `isa' hierarchy, a definition, and its immediate parent and children. The information associated with each relationship includes a unique identifier, a tree number, a definition, and the set of semantic types that can plausibly be linked by this relationship.

Obtaining the UMLS Semantic Network

The UMLS Semantic Network is provided in two formats: a relational table format and a unit record format. It is available at Distribution is subject to terms and conditions.

The Semantic Network and the other UMLS Knowledge Sources are accessible through the UMLS Knowledge Source Server ( Further information about the UMLS project can be found at

For additional information, send an email to or call1-888-FINDNLM.

A complete list of NLM Factsheets is available at:
(alphabetical list)
(subject list)

Or write to:

Office of Communications and Public Liaison
National Library of Medicine
8600 Rockville Pike
Bethesda, MD 20894

Phone: (301) 496-6308
Fax: (301) 496-4450

Last updated: 28 March 2006
First published: 28 March 2006
Metadata| Permanence level: Permanent: Stable Content
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