Myeloproliferative neoplasm, unclassifiable (MPN,U) has clinical, laboratory and morphological features of an MPN but fails to meet the criteria for any of the specific MPN entities. Because overlapping features, morphological findings in bone marrow, BCR-ABL1 fusion gene, V617F JAK2 mutation and cytogenetic abnormalities were analyzed in ten patients diagnosed with MPN,U. Bone marrow biopsy showed hypercellularity with trilineage myeloproliferation, dispersed megakaryocytes with mild pleomorphism and mature nuclei, and absence of reticulin fibrosis. All patients were BCL-ABL1 negative, while V617F JAK2 mutation was found in 6 of 8 patients. Trisomy 8 was found in two patients and t(6;12)(q12;p13) in one patient. Morphological features of MPN,U are nonspecific, however, in study cases they were most similar to diagnostic morphological features of polycythemiea vera. The high frequency of V617F JAK2 mutation in MPN,U cases analyzed revealed that its presence does not confirm a specific type of MPN.