OBJECTIVE: The prefibrotic stages of JAK2V617F essential thrombocythemia (ET) and JAK2V617F polycythemia vera (PV) can easily be diagnosed clinically without use of bone marrow biopsy histology. We assessed the 2008 WHO and European Clinical, Molecular, and Pathological (ECMP) criteria for the diagnosis of myeloproliferative neoplasms (MPNs). METHODS: Studied patients included 6 JAK2V617F-mutated ET and 4 PV patients during long-term follow-up in view of critical analysis of the literature. The bone marrow biopsy histology diagnosis without use of clinical data was PV in 7 (of which 3 were cases of ET with features of early prodromal PV) and classical PV in 4. RESULTS: The ECMP criteria distinguish 3 sequential phenotypes (1, 2, or 3) of JAK2V617F-mutated ET: normocellular ET-1; ET-2, with clinical and bone marrow features of PV (prodromal PV), and ET-3, with hypercellular dysmorphic megakaryocytic and granulocytic myeloproliferation (ET.MGM). The 3 patients with ET-2 or prodromal PV developed slow-onset PV after a follow-up of about 10 years. Bone marrow biopsy histology differentiates MPNs of various molecular etiologies from all variants of primary or secondary erythrocytoses and thrombocytoses with sensitivity and specificity of near 100%. CONCLUSION: Normocellular ET (WHO-ET), prodromal PV, and classical PV show overlapping bone marrow biopsy histology features with similar pleomorphic clustered megakaryocytes in the prefibrotic stages of JAK2V617F mutated MPN. Erythrocytes are below 6x1012/L in normocellular ET and prodromal PV, and are consistently above 6x1012/L in classical PV and at the time of transition from prodromal PV into classical PV. Red cell count at a cut-off level of 6x1012/L separates ET from PV and obviates the need for red cell mass measurement when bone marrow histology and JAK2V617F mutation screening are included in the diagnostic work-up of MPNs.