The course of essential thrombocythemia has been observed in ten patients, ages 46 to 83, of whom nine were followed for a period of 4 months to 9 years. In contrast to the experience with essential thrombocythemia recorded in the literature, manifestations of arterial thrombosis were far more common than hemorrhage. In six of the ten patients, the presenting complaints were ascribable to incipient gangrene of the toes and several of these patients additionally developed occlusion of tibial and larger arteries while under our observation. All patients with incipient gangrene showed marked clinical improvement accompanying busulfan-induced reduction and normalization of the platelet count. Relapses in five patients after 2 to 87 months responded well to retreatment with busulfan. No patient has shown evolution to another myeloproliferative disorder. Essential thrombocythemia should be considered in the differential diagnosis of occlusive arterial disease.