Elevated platelet counts may be encountered as a reactive phenomenon secondary to a variety of systemic conditions (thrombocytosis) or may represent a primary disorder of the bone marrow (thrombocythemia). The diagnosis of essential thrombocythemia is difficult and relies on exclusion of other myeloproliferative states and nonhematologic illnesses associated with increased platelet number. The paradoxic clinical complications of hemorrhage and thrombosis, the presence of splenomegaly, and the finding of various qualitative platelet abnormalities point to existence of the neoplastic disorder. Although treatment of the symptomatic patient with platelet lowering agents or antiplatelet drugs may be indicated and effective, the role of therapy in the asymptomatic individual remains highly controversial.