Objective — to carry out a correlation retrospective analysis of the histological and cytological findings of follicular cell neoplasms over a two-year period after the publication of the new WHO classification of thyroid tumors (2017), which identifies a category of follicular tumors of uncertain malignant potential (T-UMP). Material and methods. The investigators of the Clinic, National Medical Research Center for Endocrinology, Ministry of Health of Russia, made a cytohistological comparison of morphological findings, by using the material of surgically removed follicular cell neoplasms. Results. The investigation showed the lack of unreasonably common usage of the histological classification category of T-UMP (3.08%). Non-invasive follicular neoplasm with papillary-like nuclei predominantly diagnosed in the T-UMP category was more frequently (83.3) assigned to Diagnostic Category IV of the 2017 Bethesda system for the cytological diagnosis of thyroid tumors. Cytology showed the highest (98.1%) sensitivity, including classic (97.8%), encapsulated (100%), and follicular (97.7%) options in diagnosing papillary cancer, being substantially inferior in diagnosing follicular and Hürthle cell neoplasms, in which diagnostic category IV category was more frequently established for adenomas (81.4%) and carcinomas (69.5%). Conclusion. The morphological differential diagnosis of follicular cell neoplasms seems to be a relevant problem.