This is the clinical syndrome characterized by repeated spontaneous hemorrhages and a remarkable increase in the number of circulating platelets. An increased number of thrombocytes (platelets) in the blood, without a known cause. A chronic myeloproliferative disorder that involves primarily the megakaryocytic lineage. It is characterized by sustained thrombocytosis in the blood, increased numbers of large, mature megakaryocytes in the bone marrow, and episodes of thrombosis and-or hemorrhage. The cause is unknown. Median survival times of 10-15 years are commonly reported. Half of all patients are asymptomatic, but clinical presentations include thrombosis and bleeding. There are no pathognomonic features and it is a diagnosis of exclusion. Hydroxyurea is the preferred cytoreductive agent in most people with the disease. Anagrelide may be preferred in younger patients; interferon alfa 2b is preferred when treating pregnant women. The life expectancy for people with essential thrombocytosis is usually similar to that for people without the disease. Patients need CBCs on a regular follow-up basis. Low-risk asymptomatic patients do not need treatment. High-risk patients (e.g., those who are >60 years old, have thrombotic episodes, or have a platelet count >1,500,000-microliter) should be treated with aspirin and cytoreductive drugs.