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Whether or not cerebral venous thrombosis can be the first manifestation of an underlying myeloproliferative neoplasm is currently unclear. Patients with cerebral venous thrombosis were tested for the JAK2 (V617F) mutation and were followed until the development of a myeloproliferative neoplasm or censored at the end of follow-up. Ten of 152 patients (6.6%) carried the JAK2 (V617F) mutation. Three of them had known acquired risk factors for thrombosis and 5 had thrombophilia. Six patients met the diagnostic criteria for myeloproliferative neoplasm at the time of cerebral venous thrombosis, while three additional patients developed the disease during the follow-up (median duration 7.8 years, 6 months to 21.3 years), for an annual incidence of 0.26% patient-years (95% CI 0.05-0.64). The last patient has no evidence of disease after three years of follow-up. Patients without JAK2 (V617F) at the time of cerebral venous thrombosis were re-tested at the end of the follow-up and remained negative, with normal blood counts [log-rank test ?2: 159 (p