Somatic gain-of-function mutation of the Janus kinase (JAK2) gene (NM_004972.2: c.1849G>T transversion resulting in V617F) has been identified in BCR–ABL-negative chronic myeloproliferative neoplasms (MPNs) (in >95% of polycythaemia vera (PV), and in 40–60% of essential thrombocythaemia (ET) and primary myelofibrosis (PMF)). The inherited genetic background of an individual patient has been long predicted to influence disease susceptibility and phenotype in MPN. Recent studies have demonstrated that the presence of JAK2 V617F is associated with an inherited JAK2 haplotype designated as ‘46/1’ haplotype.1, 2, 3 Carriers of the 46/1 haplotype have higher risk of acquiring JAK2 V617F mutation, but the role of 46/1 haplotype as a predisposition factor for V617F-negative MPN is still remained controversial. Only a few studies4, 5 investigated the potential role of the 46/1 haplotype as a MPN phenotype modifier in the context of the frequency of complications affecting life expectancy, such as thrombosis and myelofibrotic or leukaemic transformation. To extend these observations, the aims of our study were the following: (a) to examine and confirm associations of MPN and the presence of the JAK2 46/1 haplotype in Hungarian V617F-positive and -negative MPN patients; (b) to examine associations of the 46/1 haplotype with distinct clinical characteristics of MPN, (c) to test the haplotype frequency among JAK2 V617F-negative acute myeloid leukemia (AML) patients.