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Fact Sheet
UMLS® MetamorphoSys


MetamorphoSys is the Unifed Medical Language System® (UMLS®) installation wizard and Metathesaurus® customization tool that the National Library of Medicine (NLM) updates and includes in each UMLS release. MetamorphoSys is required for local installation of the UMLS Knowledge Sources (the Metathesaurus, the Semantic Network, and the SPECIALIST Lexicon) and enables users to create customized Metathesaurus source vocabulary subsets.


Users customize their Metathesaurus source vocabulary subsets for two main purposes:

  • to exclude source vocabularies from output that are not required or licensed for a user’s local application;
  • to customize a source vocabulary subset using a variety of data output options and filters.

The Metathesaurus consists of many files, some of which are extremely large. Excluding unnecessary source vocabularies can significantly reduce the size of your subset. Given the number and variety of vocabularies contained in the Metathesaurus, it is unlikely that you will require all of its more than 150 source vocabularies. In addition, some sources require separate license agreements for specific uses, which you may not wish to obtain. Appendix 1 of the License Agreement clearly indicates which sources require separate license agreements. The Source Documentation page contains detailed information about the Metathesaurus source vocabularies.

Customization requires an understanding of the functional requirements of your specific application, your specific license arrangements, and the characteristics of relevant source vocabularies. You may need to customize the Metathesaurus by limiting vocabularies, languages, relationships, attributes or license restrictions. 

MetamorphoSys includes predefined default Metathesaurus source vocabulary subsets. You must select a default subset during initial installation. You can modify your installed default subsets to include or exclude additional source vocabularies. You may use the included browser to view your subsets after installation. To facilitate loading your subset into a local database, MetamorphoSys can generate custom load scripts for MySQL, Oracle, or Microsoft® Access.


The output of your customized Metathesaurus subset is a set of Rich Release Format (RRF) files.  You may browse or search your subset using the included RRF Browser. 

Obtaining MetamorphoSys

NLM distributes MetamorphoSys with each UMLS release, free of charge to U.S. and international users. You must accept the terms of the UMLS Metathesaurus License and create a UMLS Terminology Services (UTS) account to access the UMLS. You may download the UMLS using your UTS account.  For instructions on requesting a license and accessing the UTS, see How to License and Access the Unified Medical Language System® (UMLS®) Data. Separate license fees may apply to the use of certain Metathesaurus source vocabularies.

Using MetamorphoSys

For general information about the latest version of MetamorphoSys, see the MetamorphoSys homepage.

For details about installing and using MetamorphoSys and system requirements, see the MetamorphoSys Help page.

Other Fact Sheets in the UMLS series: UMLS Metathesaurus, UMLS Semantic Network, SPECIALIST Lexicon, UMLS Terminology Services, and Unified Medical Language System (UMLS).

For general information on NLM services, contact:

National Library of Medicine
Customer Service
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Bethesda, MD 20894
Telephone: 1-888-FINDNLM (1-888-346-3656)
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