Rayman RB. Essential thrombocythemia: aeromedical considerations. Aviat Space Environ Med 2009; 80:968–70.Essential thrombocythemia is a chronic myeloproliferative disease with possible thrombotic and hemorrhagic complications due to an abnormal increase in platelets. It is diagnosed by platelet count and the presence of increased numbers of megakaryocytes in the bone marrow. Most patients remain asymptomatic, although thrombotic events can occur in most systems of the body. A number of treatment medications are available, including hydroxyurea, anagrelide, interferon alpha, and sodium salicylate. Aeromedical disposition of aircrew engaged in civil and military aviation would depend upon the nature of operations, tolerance to the medications prescribed, and the occurrence of complications. For spaceflight, disqualification would be most prudent.