Evaluation of the Janus kinase 2 (JAK2)V617F mutation has been widely used for the diagnosis of myeloproliferative neoplasms (MPN). However, its prognostic relevance to clinical outcome is not completely understood. We investigated the association of JAK2V617F with vascular events in Korean patients with myeloproliferative neoplasms (MPN). We studied 283 patients from 15 centers, who were diagnosed with M PN. The JAK2 V617F status was evaluated by allele-specific polymerase chain reaction (PCR) and sequencing. The patients' diagnoses were essential thrombocythemia (ET n=146), polycythemia vera (PV n=120), primary myelofibrosis (n=12), and unclassifiable MPN (MPNu n=5). JAK2V617F was detected in 89 (61%) patients with ET 103 (86%) with PV, four (33%) with myelofibrosis, and four (80%) with MPNu. A higher number of leukocytes, haemoglobin levels and BM cellularity as well as an older age, lower platelet counts, and diagnosis of PV were significantly correlated with JAK2 V617F. Eighty-three and 43 episodes of thrombosis and bleeding occurred in 100 patients each before and after the diagnosis-Vascular events more frequently occurred in 37% of patients with JAK2 V617F than in 29% of those without the mutation (p=0.045). Among 175 patients whose samples were available for sequencing, 28 patients with homozygous JAK2 V617F had vascular events more frequently (57%) than those who were heterozygotes (39%) or had the wild type (27%) (p=0.03). The multivariate analysis showed that a JAK2 homozygous mutation, hypercholesterolemia and older age were independent risk factors for a vascular event. The results of this study showed that Korean patients with MPN had a similar JAK2 mutation rate and frequency of vascular events when compared to Western patients. The presence of V617F was significantly related to vascular events. Therefore, initial evaluation for the JAK2 mutation and careful monitoring for vascular events should be performed in MPN patients.