Polycythaemia vera is a clonal myeloproliferative disorder mainly involving proliferation of the erythropoiesis. The symptoms are varied and usually unspecific. The most serious complications of the disease are a thrombotic tendency and myeloid metaplasia. Thromboses mainly (2/3) occur in the arterial and less often (1/3) in the venous system and are the most common cause of death. Criteria have been established for the diagnosis of polycythaemia vera which should be strictly observed in order to distinguish between this disease, spurious polycythaemia, essential thrombocythaemia and secondary erythrocytosis. The goal of treatment is the prevention of thromboembolic complications and of myeloid metaplasia. The initial treatment always consists of phlebotomies until a hematocrit < 45% is achieved. The decision regarding maintenance therapy is difficult. While hydroxyurea or radiophosphorus is the treatment of choice in older patients (> 70 years of age), it is more difficult to select the appropriate therapy for younger patients (< 60 years), since recent studies have indicated that hydroxyurea treatment might increase the risk of leukemia. Interferon is effective but its role in the treatment of polycythaemia vera has not been established.