Chronic myeloid neoplasms (CMPN) consists of three main entities, polycythemia vera (PV), essential thrombocythemia (ET) and primary myelofibrosis (PMF). These three disorders have many overlapping clinical features. The diseases are clonal stem cell disorders characterized by a chronic excess production of mainly mature myeloid cells. The excess production of clonal red cells (in PV), platelets (in PV, ET and PMF) and leukocytes (mainly PV and PMF)leads to a highly increased risk of thrombosis. Patients may also suffer from constitutional symptoms, pruritus and splenomegaly. An inherent feature of these diseases are the risk of ET and PV of transformation to myelofibrosis and a risk of both ET, PV and PMF of leukemic transformation.