Recently, V617F mutation in JAK2 tyrosine kinase gene was established as a marker of myeloproliferation, useful for proving clonality and securing diagnosis in a considerable proportion of the myeloproliferative neoplasms (MPN) The discovery presents a major breakthrough in the understanding of the pathogenesis of the MPN. Moreover, some studies suggest a possible role of the JAK2V617F mutation in the pathogenesis of some specific acute myeloid leukemia (AML) subtypes. To further improve the understanding of the role of JAK2V617F mutations in the pathogenesis and the clinical course of the myeloid malignancies we screened 192 patients with various MPN and AML for the mutations and analyzed the possible association between JAK2V617F mutations and the clinical features of MPNs patients. The frequency of V617F JAK2 mutation was analyzed by theallele-specific PCR assay. Out of 153 cases with known or suspected diagnoses of MPNs, 100 (65.3%) were positive for the JAK2V617F mutation and 53 (34.7%) were negative. In 39 AML cases the mutant allele V617F was not expressed. Correlations of the clinical features at diagnosis and long-term prognosis between the two JAK2-V617F different MPNs groups revealed comparability regarding all tested parameters except for the incidence of thrombotic history. Patients with the mutation had significantly higher incidence of thrombotic complication (38.5%), compared to the group without the mutation (19.2%) (P < 0.005). Our results confirmed the diagnostic significance of JAK2V617F mutation in MPNs and supported the notion that patients with the mutation should be classified in a new entity of MPNs.