Chronic myeloproliferative neoplasms (MPNs) are a group of related conditions characterized by the overproduction of cells from one or more myeloid lineages. More than 95% of cases of polycythemia vera, and roughly half of essential thrombocythemia and primary myelofibrosis acquire a unique somatic 1849G>T JAK2 mutation (encoding V617F) that is believed to be a critical driver of excess proliferation. We report here that JAK2(V617F)-associated disease is strongly associated with a specific constitutional JAK2 haplotype, designated 46/1, in all three disease entities compared to healthy controls (polycythemia vera, n = 192, P = 2.9 x 10(-16); essential thrombocythemia, n = 78, P = 8.2 x 10(-9) and myelofibrosis, n = 41, P = 8.0 x 10(-5)). Furthermore, JAK2(V617F) specifically arises on the 46/1 allele in most cases. The 46/1 JAK2 haplotype thus predisposes to the development of JAK2(V617F)-associated MPNs (OR = 3.7; 95% CI = 3.1-4.3) and provides a model whereby a constitutional genetic factor is associated with an increased risk of acquiring a specific somatic mutation.