An underlying myeloproliferative disorder (MPD), especially polycythemia vera (PV) or essential thrombocythemia (ET), is a risk factor for thrombosis. Considering large selected studies, prevalence rates for major thrombosis, at time of diagnosis, range from approximately 34 to 39% for PV and 10 to 29% for ET; the corresponding figures for thrombosis at follow-up are approximately 8 to 19% for PV and 8 to 31% for ET. In all instances, arterial events were more frequent than venous events. In both PV and ET, advanced age and history of thrombosis are independent predictors of recurrent thrombosis. In addition, leukocytosis, but not thrombocytosis, has been identified as a potential risk factor for thrombosis in both diseases. The particular observation is consistent with the laboratory demonstration, in these disorders, of increased number of activated granulocytes and granulocyte-platelet aggregates, upregulation of platelet P-selectin and tissue factor expression by granulocytes, and the antithrombotic value of hydroxyurea therapy. Most recently, a JAK2 gain-of-function mutation ( JAK2V617F) was described in virtually all patients with PV and approximately 50% of those with ET. Whether the presence of this specific mutation or its allele burden modifies the risk of thrombosis in patients with MPDs currently is under investigation.