Chronic myelocytic leukemia (CML) is a well-known myeloproliferative disorder, with typical clinical and hematological features, which develops into a lethal blastic crisis within an average of 4 years. In the last few years much has been learned about the cell responsible for the leukemic deviation, chromosomal abnormalities both in chronic phase and in blastic transformation, biochemical and cytochemical behavior of leukemic cells, and the immunology of the disease. On the contrary, polychemotherapy has brought but little progress in CML treatment. However, thorough research on prognostic factors at the onset of the disorder allows a more rational approach to the treatment, and in the near future interesting progress in bone marrow transplantation is to be expected. Also, monoclonal antibody techniques will reveal further knowledge on the origin and differentiation of leukemic cells and the relationship between the chronic phase and the blastic deviation.