Essential thrombocythaemia is one of the chronic myeloproliferative neoplasms and is characterised by a sustained platelet count > or = 450 x 10(9)/l. The clinical course is often indolent but a significant number of patients have thrombotic and haemorrhagic complications. A patient with neurological complications is described, and the pathogenesis and clinical features of this disease are reviewed, including its neurological sequelae and management, along with an outline for the investigation of patients with thrombocytosis.