In general, megakaryocytic myelosis is nowadays considered to be a separate disease entity, one of the myeloproliferative syndromes. Morphologically there are localised or diffuse proliferations of usually large pleomorphic megakaryocytes and immature atypical megakaryocytes up to megakaryoblasts in the bone marrow, in the sense of a haemoblastosis. In the course of the disease megakaryocytic splenomegaly develops. A sarcomatous form (megakaryoblastoma, megakaryo-sarcoma) is rare. Megakaryocytic myelosis may arise from chronic meyloid leukaemia or polycythaemia vera, rarely as a transitional stage to an acute myeloblastic leukaemia or megakaryoblastic leukemia in the sense of a blast crisis. The mature form of the disease, which has an age peak at 59 years and is not sex-linked, often takes a course over years with increasing splenomegaly, anaemia, moderate leucocytosis and usually marked thrombocytosis (average value of 720 X 10(9)/1). Life threatening complications are haemorrhages, thromboembolism and increased frequency of infections due to antibody deficiency in the advanced stage.