The chronic myeloproliferative syndromes are bone marrow stem cell disorders. An increase of cell counts of one or rather all three blood cell types is characteristic for these disorders. The most important diseases in this group are: chronic myelogenous/granulocytic leukemia, polycythemia rubra vera, osteomyelosclerosis or agnogenic myeloid metaplasia and essential thrombocythemia. The cells are normally differentiating in these diseases, while the control of cell dividing is abnormal and therefore the cells are produced and accumulated in excess. Splenomegaly is a common and characteristic clinical finding. The fibrotic or sclerotic transformation of the bone marrow can take place in all forms of the syndrome. Extramedullary haematopoiesis can occur in all of the above diseases, but it is most common in myelofibrosis/agnogenic myeloid metaplasia. In the last phase of the disease a terminal blastic crisis may terminate the course of chronic myeloproliferative diseases. The myeloproliferative disorders can be transformed in each other--the most common transformation is that of the polycythemia rubra vera into myelofibrosis. The greatest progress in the therapy of chronic myeloproliferative diseases is achieved in chronic myelogenous leukemia: bone marrow transplantation and interferon treatment (the latter also in essential thrombocythemia and polycythemia rubra vera) are routine modalities worldwide. A new drug, anagrelide is effective in the treatment of myeloproliferative thrombocytosis and thrombocythemia.