The spectrum of thrombotic manifestation in ET and PV. Thrombophilia, which severely affects the morbidity and mortality of PV and ET, is variably characterized by microcirculatory disturbances and arterial and venous thromboses that often precede disease recognition. Thrombotic occlusions of large arteries most commonly involve cerebral or coronary vessels. Ischemic stroke constitutes 30%-40% of all thrombotic events in PV patients. Acute coronary syndromes are more rare, particularly during followup of treated patients. Acute vascular occlusions in other areas are not uncommon both in PV and ET patients. Venous thrombosis usually manifests with an increased incidence of deep venous thromboses of the lower limbs, which may cause pulmonary embolism. Superficial phlebitis of the legs is also common and venous thromboses at unusual sites are not as rare as in the general population. Thromboses of cerebral sinuses and of splanchnic (portal and hepatic) veins have been repeatedly reported in relatively young female patients. Microcirculatory disturbances are the most peculiar thrombotic manifestations in PV and ET patients and are responsible for a wide range of clinical symptoms arising from the formation of platelet thrombi in the end-arterial circulation of the peripheral, cerebral, coronary, skin, and abdominal vessels.