Essential thrombocythemia is a chronic myeloproliferative disease characterized by persistent thrombocytosis and an increased risk of thromboembolic complications. Most patients are asymptomatic at the time of diagnosis. In this article guidelines for diagnosis and treatment of this disorder are presented on behalf of the Norwegian Society of Hematology. On the basis of a literature search in international databases (Medline) and international medical journals, articles have been selected according to their clinical relevance. The risk of major thrombosis is higher in patients older than 60 years and highest among those with a previous occlusive event. In individual patients there is no clear relationship between platelet count and risk of thrombosis. Low-dose aspirin (75-100 mg/day) is recommended to all patients with platelet count < 1,000 x 10(9)/l and in the absence of previous bleeding episodes. All patients with disease-related complications and asymptomatic patients with platelet count above 1,000-1,500 x 10(9)/l should receive cytoreductive treatment. Hydroxyurea is recommended for patients over 60 years of age. In younger patients, potential leukemogens should be avoided, and anagrelide or interferon-alpha is recommended.