Thrombophilia, which severely impacts on morbidity and mortality of polycythaemia vera and essential thrombocythaemia, is variably characterized by microcirculatory disturbances, arterial and venous thromboses that often precede disease recognition. Thus, the search for Janus Kinase 2 mutation, the molecular marker of myeloproliferative neoplasms, is becoming increasingly common particularly in patients with vein thromboses at atypical sites. Although the pathogenesis of thrombophilia is still elusive, platelet and leukocyte abnormalities seem particularly critical and likely account for the antithrombotic efficacy of aspirin and hydroxyurea.