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2025AB UMLS® Release Notes and Bugs

What's New

The 2025AB release of the Unified Medical Language System® (UMLS) Knowledge Sources is available for download as of November 2025. Access it on the UMLS Download page at https://www.nlm.nih.gov/research/umls/licensedcontent/umlsknowledgesources.html.

To access the UMLS Release files, you must have an active UMLS Metathesaurus® License and a valid UTS account (https://uts.nlm.nih.gov/uts/).  Upon download, you will be prompted to authenticate with an identity provider with the UTS.

Available downloads: UMLS Metathesaurus Precomputed Subsets
In past years, we have required MetamorphoSys to install and customize the UMLS Metathesaurus. We are now providing ready-to-use precomputed subsets of the UMLS Metathesaurus that require no installation or customization. As of 2024AB, precomputed subsets include obsolete and suppressible content. UMLS Metathesaurus History Files
The following files contain historical data from the UMLS Metathesaurus starting with the 2004AA release: If you find these files useful, please provide feedback to the NLM Help Desk (https://support.nlm.nih.gov/support/create-case/) with the subject line: "UMLS History Files".

Metathesaurus
The 2025AB Metathesaurus contains approximately 3.49 million concepts and 17.39 million unique concept names from 190 source vocabularies.

Source update: Content View removed: 32 English sources and 43 translation sources were updated.  These include MeSH, LOINC, RxNorm, and SNOMED CT.  For detailed information on changes in this version of the Metathesaurus, see the Updated Sources (Expanded) section.  Additional release statistics may be found in the Statistics section.

MetamorphoSys
The full release requires about 35 GB of disk space. MetamorphoSys can generate custom load scripts for MySQL, Oracle, or Microsoft Access when creating a Metathesaurus subset or installing the Semantic Network. Instructions are available on the UMLS Load Scripts homepage.

UMLS Learning Resources
2025AB Source Release Documentation Web pages are available. Additional information about the UMLS is available on the UMLS homepage (https://www.nlm.nih.gov/research/umls/index.html). New users are encouraged to explore the UMLS FAQ and other training materials.

UMLS Terminology Services (UTS)
The UMLS Metathesaurus Browser and the UMLS API have been updated with the most recent release. 

To authenticate with the UMLS API, append an apiKey parameter to your request, for example: 
https://uts-ws.nlm.nih.gov/rest/content/current/CUI/C0155502/atoms?apiKey={{YOUR_API_KEY}}

The UMLS Metathesaurus Browser was updated to feature additional relationships between UMLS concepts. Whereas we previously displayed only broader and narrower relationships for each UMLS concept, we now feature other useful relationships found in the UMLS Metathesaurus such as “classifies” (MDR), “has_finding_site” (SNOMEDCT_US), and more. To see an example, go to https://uts.nlm.nih.gov/uts/umls/concept/C0002395 and click the Related Concepts tab.
Screenshot of UMLS Terminology Services browser feature displaying related concepts


UMLS User Contributions
UMLS users have extended the functionality of the UMLS by developing APIs, automation scripts, and natural language processing tools. You can find a list of these on the UMLS Community web page: https://www.nlm.nih.gov/research/umls/implementation_resources/community/index.html. 
Want to add your tool? Send a request to the NLM Help Desk (https://support.nlm.nih.gov/support/create-case/) with the subject line: "UMLS Community". Be sure to include a link to your source code so that other UMLS users can adapt your tool. We are especially interested in: We value your feedback! For more information about improvements made to the UMLS based on user feedback, as well as information about UMLS usage, see our UMLS User Feedback Page: https://www.nlm.nih.gov/research/umls/implementation_resources/community/user_feedback.html
There are no known problems for the 2025AB release. Please send any issues or concerns to NLM Customer Service.

For information on the Unified Medical Language System (UMLS), consult the UMLS homepage at: https://www.nlm.nih.gov/research/umls.

The latest Release Notes list all known issues, including bugs and fixes, and are available at: https://www.nlm.nih.gov/research/umls/knowledge_sources/metathesaurus/release/notes.html.

UMLS data files and MetamorphoSys are available by download from the UMLS Web site at: https://www.nlm.nih.gov/research/umls/licensedcontent/umlsknowledgesources.html. Users must have an active UMLS Terminology Services (UTS) account to download the files. All files must be downloaded into the same directory.  MetamorphoSys must be unzipped before it can be used.

Bugs

There are no known problems for this release. 

Please send any issues or concerns to NLM Customer Service.

UMLS Problems (Bugs) for Previous Versions

Release information for the RRF Browser

Last Reviewed: November 17, 2025