Abstract： Objective To investigate the frequency of JAK2V617F mutation in Chinese patients with chronic myehiproliferative neoplasms (MPN) and to study the relationship between JAK2V617F mutation and clinical characteristics. Methods JAK2V617F mutation was screened by allele-specific polymerase chain reaction (AS-PCR). Results JAK2V617F mutation was detected in 277 of the 412 patients with MPN. The frequency of JAK2V617F mutation was similar among essential thrombocythernia (ET), idiopathic myelofibrosis (IMF) and chronic myeloproliferative diserders-unelasaified (MPD-U) (P >0.05), but it was significantly lower than that in polycythemia vera (PV) (P <0.05). The presence of JAK2V617F was found to be significantly correlative with advanced age at diagnosis (P <0.01) and with higher hemoglobin levels and higher leukocyte counts (P < 0.05). Significant difference was found in complication of vascular events between JAK2V617 positive and negative patients (P <0.05). JAK2V617F positive MPD-U patients were more prone to progress into typical MPN compared with JAK2V617F negative MPD-U patients. The association between abnormal karyotype and JAK2V617F was not found in cytogenetical analysis of 301 patients. Conclusion The presence of JAK2V617F in MPD-U is associated with the disease development. There is a correlation between JAK2V617F mutation in MPN and advanced age, higher leukocyte counts, hemoglobin level and vascular events.