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Unified Medical Language System® (UMLS®)

2021AB UMLS® Release Notes and Bugs

What's New

The 2021AB release of the Unified Medical Language System® (UMLS) Knowledge Sources is available for download as of November 1, 2021.

The available downloads are:

  • Full Release (UMLS Metathesaurus, Semantic Network, Specialist Lexicon and Lexical Tools, database load scripts, and MetamorphoSys for customizing your UMLS subset and browsing the data)
  • UMLS Metathesaurus Files (complete Metathesaurus data without any customization)
  • MRCONSO.RRF file (most widely used Metathesaurus file)

The SOAP API was discontinued on December 31, 2020. SOAP API users are encouraged to switch to the REST API (


The 2021AB Metathesaurus contains approximately 4.54 million concepts and 16.5 million unique concept names from 220 source vocabularies.

One new source:

  • CDCREC (CDC Race and Ethnicity Code Set)

One new NCI sub-source:

  • NCI_CCPS (Childhood Cancer Predisposition Study Terminology)

One mapping removed:

  • LNC215 to CPT2005 Mapping

74 English sources and 33 translation sources were updated.  These include MeSH®, MedDRA, RxNorm, and SNOMED CT® (English and Spanish).  A complete list is available in the Updated Sources section of the Release Documentation.  For more 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.


  • The full release requires 33.4 GB of disk space.

Reported bugs may be viewed on the Release Notes and Bugs Web page.

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.

Release Information
To access the UMLS Release files, you must have an active UMLS Metathesaurus® License and a valid UTS account.  You will be prompted to authenticate with an identity provider with the UTS when downloading the files.

UMLS Learning Resources
2021AB Source Release Documentation Web pages are available.

Additional information regarding the UMLS is available on the UMLS homepage. New users are encouraged to take the UMLS Basics Tutorial and to explore the UMLS Quick Start Guide, and other training materials.

UMLS Terminology Services (UTS).
The UTS Metathesaurus Browser and the UTS REST API include the updated release.

UMLS User Contributions
UMLS users have extended the functionality of the UMLS in a variety of ways by developing APIs, automation scripts, and natural language processing tools. You can find a list of these on the UMLS Community web page:

Want to add your tool? Send a request to the NLM Help Desk ( 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:

  • Database load scripts
  • Transformation scripts that convert UMLS data into other formats (for example, RDF or JSON)
  • Scripts that automate any aspect of UMLS installation
  • Applications that leverage UMLS in the processing of text


Title: Issues with Version 2.71 of LNC-DE-DE

Problem: The 2021AB version of the UMLS Metathesaurus includes version 2.71 of LNC-DE-DE (LOINC Linguistic Variant - German, Germany). LOINC found issues with this version and reverted to version 2.70. For the most current versions of LOINC Linguistic Variants, see:

Solution: This error will be corrected in the 2022AA Release.

Date Posted: November 1, 2021

Please send any issues or concerns to NLM Customer Service.

UMLS Problems (Bugs) for Previous Versions

Release information for the RRF Browser

Last Reviewed: October 29, 2021