Background: The rapid growth in the volume of medical data, the extensive possibilities of information technology, the transfer of medical document flow to electronic format generates a high demand for the introduction of information and reference assistance tools and clinical decision support systems (CDSS). Work on the creation of CDSS currently combines the expert activities of doctors with the work of information technology specialists, mathematical statisticians, data scientists, knowledge engineers. Most of the developments involving the formation of knowledge bases are created in isolation, without the use of universal approaches that allow combining various solutions. At the heart of any medical knowledge base (MKB) there is a thesaurus, which is a systematized dictionary of terms that helps to standardize terminology, which makes it possible to speed up the search and exchange of information. It includes concept terms and relationships between them, as well as synonyms and various attributes. Aims — creation of a national medical metathesaurus, built on the ontological principle and the development of MKB based on it. Methods. International systematized dictionary of medical terms UMLS (Unified Medical Language System); clinical recommendations for 22 groups of nosologies; reference books of the federal portal of normative reference information of the Ministry of Health of the Russian Federation; electronic medical records – 330 thousand (dataset MIMIC-IV); abstracts of PubMed publications-28 million. Semantic analyzers SemRep (Semantic Repository) and MetaMap were used; methods for evaluating lexical similarity, connectivity, contextual combinability of entities in a subgraph, and mathematical statistics. Results. The first version of the Unified National Medical Nomenclature (UNMN) has been created. It is proved that ontological models are an effective way of presenting structured information. Components of information search engines have been created. Analytical tools for working with metathesaurus have been developed. Conclusions. On the basis of the UNMN and the created tools, it is possible to automate the formation of a clinical picture of the disease (knowledge base) and single-platform development of the CDSS.