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The Unified Medical Language System (UMLS) is a set of files and software, maintained by the U.S. National Library of Medicine (NLM), that brings together many health and biomedical vocabularies and standards to enable interoperability between computer systems. For clinical researchers and informaticians, it is the NLM-distributed way to link terms and codes from vocabularies such as SNOMED CT, LOINC, MeSH, RxNorm and ICD-10-CM through shared concept identifiers.
What It Is: Unified Medical Language System (UMLS)
What is the Unified Medical Language System (UMLS)? NLM describes it as a resource that integrates and distributes key terminology, classification and coding standards, and associated resources to promote more effective and interoperable biomedical information systems and services, including electronic health records. Rather than replacing the source vocabularies, the Metathesaurus preserves the meanings, concept names and relationships from each vocabulary while linking similar names for the same concept.
In multi-site research, one system may record laboratory results with LOINC codes while another records diagnoses with ICD-10-CM. NLM lists linking terms and codes across systems and facilitating mapping between terminologies among the uses of the UMLS, which is why it often appears in data integration work on clinical datasets. Teams planning studies can also read about clinical trial protocol design and good clinical practice for the surrounding requirements.
Who Runs It and What It Covers
The UMLS is produced by the National Library of Medicine, part of the National Institutes of Health. Releases come twice a year: the AA release in May and the AB release in November. The 2026AB statistics page lists 3,550,208 concepts, 18,302,757 concept names (AUIs) and contributions in 31 languages, with source vocabularies counted as 177 distinct source families. NLM describes the Metathesaurus as linking similar names for the same concept from nearly 200 vocabularies.
When people ask what database was developed by the National Library of Medicine, the UMLS Metathesaurus is one answer; NLM also produces PubMed/MEDLINE and the Medical Subject Headings (MeSH), which are separate products. The UMLS itself has three Knowledge Sources:
- The Metathesaurus: The biggest component of the UMLS. What is UMLS Metathesaurus? It is a large biomedical thesaurus organized by concept, or meaning, that links similar names for the same concept from many vocabularies and identifies useful relationships between concepts. Each concept has a Concept Unique Identifier (CUI), such as C0015967. The release statistics also count atoms (AUIs), string identifiers (SUIs) and normalized-string identifiers (LUIs).
- The Semantic Network: Broad categories (semantic types) and the relationships between them. NLM states that there are 133 semantic types and that every Metathesaurus concept is assigned at least one. This categorization is useful background for knowledge graph work in biomedical informatics.
- The SPECIALIST Lexicon and Lexical Tools: A large syntactic lexicon of biomedical and general English, plus tools for normalizing strings, generating lexical variants and creating indexes.
How to Search or Use It
NLM’s UMLS Terminology Services (UTS) provides three ways to access the data: web browsers, local installation and web service APIs. The usual sequence, taken from the NLM pages listed under Sources, is:
- Request a license and a UTS account: Sign up through the UMLS Terminology Services site. NLM states that UMLS licenses are issued only to individuals, not to groups or organizations, and that there is no charge for licensing the UMLS from NLM.
- Browse in the UTS applications: The Metathesaurus Browser retrieves concept information, including CUIs, semantic types and synonymous terms. The Semantic Network Browser shows the names, definitions and hierarchy of the Semantic Network.
- Get your API key: The UMLS API documentation links to a “Get Your API Key” page for programmatic access.
- Search the REST API: The documented base URI is
https://uts-ws.nlm.nih.gov/rest, and the path/search/{version}searches by term or code and returns CUIs or source-asserted identifiers. - Retrieve concept content:
/content/{version}/CUI/{CUI}retrieves information about a known CUI, and/content/{version}/CUI/{CUI}/atomsretrieves its atoms, which is the basis for crosswalking between vocabularies. - Install locally if you need to: Download the files; the MetamorphoSys tool included with the download lets you customize the UMLS, after which you can load the data into your own database system such as MySQL or Oracle, or browse it with the MetamorphoSys RRF browser.
Strengths and Limits
- Strengths:
- Links many vocabularies, including CPT, ICD-10-CM, LOINC, MeSH, RxNorm and SNOMED CT, while preserving each source’s concept names and relationships.
- No charge for licensing the UMLS from NLM, and no charge for SNOMED CT use in the United States and other SNOMED International member countries.
- Three access routes (browsers, local installation, APIs) and a twice-yearly release cycle.
- Limits:
- Licensing is per individual, and NLM notes that some uses may require additional agreements with individual terminology vendors. The release statistics group atoms by Source Restriction Level (0, 1, 2, 3, 4 and 9), so check each source’s level before reuse.
- Local installation means loading and managing a very large database (millions of concepts and tens of millions of names).
- Because sources were designed for different purposes, such as billing versus clinical documentation, mappings between them need review rather than blind trust.
Alternatives to UMLS
Other terminology resources overlap with the UMLS in scope. Terms of use differ, so confirm them on each project’s own site.
| Resource | Maintained by | Primary scope | Typical use |
|---|---|---|---|
| UMLS | U.S. National Library of Medicine | Links terms and codes from many biomedical vocabularies through CUIs | Mapping between terminologies, text processing, building local terminology services |
| SNOMED CT | SNOMED International | Clinical healthcare terminology (one of the vocabularies inside the UMLS) | Clinical documentation in electronic health records |
| OBO Foundry | Open Biological and Biomedical Ontologies community | Reference ontologies for the life sciences | Biological research and annotation |
| OHDSI Athena | Observational Health Data Sciences and Informatics | Standardized vocabularies for the OMOP common data model | Multi-site observational health data studies |
Hospital quality-reporting teams often work with coded clinical data; see our guide to eCQM reporting for hospitals for that context.
Frequently Asked Questions
What is the Unified Medical Language System (UMLS)?
The UMLS is a set of files and software from the National Library of Medicine that brings together many health and biomedical vocabularies and standards to enable interoperability between computer systems.
What database was developed by the National Library of Medicine?
The UMLS Metathesaurus is NLM’s multi-vocabulary concept database. NLM also produces separate resources such as PubMed/MEDLINE and the Medical Subject Headings (MeSH).
Is UMLS free?
NLM states there is no charge for licensing the UMLS, but you need a license and a UMLS Terminology Services account, and licenses are issued only to individuals. No charge also applies to SNOMED CT use in the United States and other member countries. Some uses may require additional agreements with individual terminology vendors.
What is UMLS Metathesaurus?
It is the biggest component of the UMLS: a large biomedical thesaurus organized by concept that links similar names for the same concept from nearly 200 vocabularies and preserves each vocabulary’s meanings, concept names and relationships.
What is a UMLS CUI?
A CUI is a Concept Unique Identifier in the Metathesaurus. NLM’s REST documentation uses C0015967 as its example CUI, which the Metathesaurus Browser and the API can resolve to concept information.
Sources
All pages below were checked on November 7, 2026.
- National Library of Medicine. Unified Medical Language System (UMLS). https://www.nlm.nih.gov/research/umls/index.html.
- National Library of Medicine. UMLS Metathesaurus. https://www.nlm.nih.gov/research/umls/knowledge_sources/metathesaurus/index.html.
- National Library of Medicine. Metathesaurus Statistics, 2026AB. https://www.nlm.nih.gov/research/umls/knowledge_sources/metathesaurus/release/statistics.html.
- National Library of Medicine. UMLS Terminology Services REST API documentation. https://documentation.uts.nlm.nih.gov/rest/home.html.
- National Library of Medicine. UMLS Terminology Services license category help. https://uts.nlm.nih.gov/uts/license-category-help.
- Bodenreider O. The Unified Medical Language System (UMLS): integrating biomedical terminology. Nucleic Acids Research. 2004;32(Database issue):D267-D270. https://doi.org/10.1093/nar/gkh061.
