Skip to Content
United States National Library of Medicine National Institutes of Health

Fact Sheet
UMLS ® Knowledge Source Server


The Unified Medical Language System® (UMLS) approach involves the development of a set of widely distributed Knowledge Sources (Metathesaurus®, Semantic Network, and SPECIALIST Lexicon) that can be used by a variety of applications to compensate for differences in the way concepts are expressed in a variety of computerized biomedical sources.

The UMLS Knowledge Source Server (UMLSKS) is a computer application that provides Internet access to the Knowledge Sources and other related resources made available by developers using the UMLS. Its purpose is to make UMLS data more accessible to users, and in particular to system developers. The system architecture allows remote site users (individuals as well as computer programs) to send requests to a server at the National Library of Medicine (NLM). Access to the system is provided through the World-Wide Web, an Extensible Markup Language (XML)-based socket programming interface, and through an Application Programmer Interface (API).


The UMLSKS allows the user to request information about particular Metathesaurus concepts, including attributes such as the concept's definition, its semantic types, concepts that are related to it, hierarchical context details, co-occurrence information from MEDLINE® and AI-RHEUM, etc., all of which can be restricted to source specific details. The UMLSKS also allows the user to request information about the attributes themselves, for example, by asking for all the concepts that have been assigned to a particular semantic type, or by asking for all the terms that have a particular lexical tag.

Semantic Network

The Semantic Network contains information about semantic types and their relationships. The implementation of the network module computes the relationships between semantic types using the inheritance property of the network type hierarchy. Information in the Semantic Network can be queried in terms of two semantic types and the relationship between them. Individual queries are specified by providing the known types or relations and leaving out the unknowns. The system then retrieves the corresponding values for the unknowns. If the user wishes to know what types are related by a particular relation, then the user would indicate only the relationship name and all the semantic type pairs linked by that relationship would be retrieved. The user might also wish to know if a particular relationship holds between a pair of types.


The UMLSKS also provides access to lexical records in the SPECIALIST Lexicon. The lexicon entry for each word, or term, records syntactic, morphological, and orthographic information. Lexical entries may be single or multi-word terms. Lexical information includes syntactic category, inflectional variation (e.g. singular and plural for nouns, the conjugation of verbs, the positive, comparative, and superlative for adjectives and adverbs), and allowable complementation patterns (i.e., the objects and other arguments that verbs, nouns, and adjectives can take).

UMLS Resources

A number of additional resources for use by developers and researchers are available through the UMLSKS. Source files for the three Knowledge Sources may be downloaded from the UMLSKS. Source code and documentation for the SPECIALIST Lexicon programs are also made available to developers.

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: 06 May 2004
First published: 05 November 2002
Metadata| Permanence level: Permanent: Stable Content