Migraine-like cerebral transient ischemic attacks (MIAs) and ocular ischemic manifestations were the main presenting features in 10 JAK2V617F-positive patients studied, with essential thrombocythemia (ET) in 6 and polycythemia vera (PV) in 4. Symptoms varied and included cerebral ischemic attacks, mental concentration disturbances followed by throbbing headaches, nausea, vomiting, syncope or even seizures. MIAs were frequently preceded or followed by ocular ischemic events of blurred vision, scotomas, transient flashing of the eyes, and sudden transient partial blindness preceded or followed erythromelalgia in the toes or fingers. The time lapse between the first symptoms of aspirin-responsive MIAs and the diagnosis of ET in 5 patients ranged from 4 to 12 years. At the time of erythromelalgia and MIAs, shortened platelet survival, an increase in the levels of the platelet activation markers β-thromboglobulin and platelet factor 4 and also in urinary thromboxane B2 were clearly indicative of the spontaneous in vivo platelet activation of constitutively JAK2V617F-activated thrombocythemic platelets. Aspirin relieves the peripheral, cerebral and ocular ischemic disturbances by irreversible inhibition of platelet cyclo-oxygenase (COX-1) activity and aggregation ex vivo. Vitamin K antagonist, dipyridamole, ticlopidine, sulfinpyrazone and sodium salicylate have no effect on platelet COX-1 activity and are ineffective in the treatment of thrombocythemia-specific manifestations of erythromelalgia and atypical MIAs. If not treated with aspirin, ET and PV patients are at a high risk of major arterial thrombosis including stroke, myocardial infarction and digital gangrene.