Essential thrombocythaemia (ET) is a rare clonal myeloproliferative disorder characterized by a persistent increase in platelet count. The commonly used criteria for the diagnosis, except for the level of the increase in platelet count, are usually those fixed by the Polycythemia Vera Study Group. The average age of onset is around 60 years, both sexes being affected. The symptoms frequently present at diagnosis are related to microcirculatory disturbances (palms, soles, fingers). Neurological symptoms are often observed. Thrombotic complications of the large vessels are less frequent. Haemorrhagic problems are present in about 30% of patients. Bleeding time is normal in most cases, whereas platelet aggregation abnormalities are frequently found. Nil adrenaline aggregation is the most discriminative test. The clinical course is characterized by long intervals without any symptoms; thromboembolic or haemorrhagic episodes can, however, occur, mainly in uncontrolled ET. Development of terminal acute leukaemia has been reported in 34 cases. The expression of the influence of the treatments, 32P or alkylating agents, is very strong. The treatment of ET has to take in consideration the difficult compromise between balancing the necessity of preventing complications and the effects of drug toxicity. The use of recombinant alpha-interferon has recently been proposed and is under investigation. The pathogenesis of thrombocytosis in ET seems to involve an expansion in the megakaryocyte progenitor cell pool. Platelet membrane glycoprotein abnormalities and defective glycosylation of thrombospondin have been shown. Numerous other platelet abnormalities, including decreased alpha-adrenergic receptors, loss of PGD2 receptors and increased Fc receptors, have been reported. Arachidonic metabolism seems to be abnormal and lipoxygenase is defective. Most of the platelet abnormalities seem to be the result of intrinsic defects at the level of an abnormal clone of megakaryocytes. However, causal relationships between the platelet abnormalities and bleeding or thrombosis are not yet clearly demonstrated.