C0000959

The Unified Medical Language System is the NLM's meta-terminology that unifies SNOMED CT, ICD-10, ICD-11, RxNorm, LOINC, MeSH, and around 200 other source vocabularies under a single concept identifier (CUI). One UMLS CUI can group dozens of synonymous codes from different terminologies.

A CUI is a stable identifier (like C0011860) that points at a single clinical idea regardless of how that idea is encoded in any specific vocabulary. CUIs are the most reliable way to bridge between terminologies when you need to compare or merge data from multiple sources.

Every UMLS concept page lists the codes it groups together. That gives you a free crosswalk: a SNOMED concept and an ICD-10 code that share a CUI describe the same clinical idea. Our cross_references API returns these groupings inline.

Browsing the directory is free. Programmatic /v1/reference/umls/* access needs an API key. Production volume requires a paid tier.

We track NLM's twice-yearly releases (May AB and November AB). Each release brings new concepts, retirements, and atom updates from the source vocabularies, and our index reflects them within the same week.

Yes. The UMLS Metathesaurus License Agreement is free but requires an NLM UTS account. Customers in production need their own license. Free-tier evaluation use is permitted under the standard terms.