61 cases of chronic granulocytic leukemia were reviewed in this paper. Based on the results, the following features are suggested as the diagnostic criteria for early acute transformation: (1) fever of unknown cause, (2) bone and joint pain, (3) a fall in hemoglobin, marked increase or decrease of the number of WBC's and platelets, (4) myeloblasts and promyelocytes exceed 5% in peripheral blood and 10% in bone marrow, and (5) poor or no response to previously effective drug therapy, It is also suggested that a rational therapeutic management in the chronic phase may help to prevent or delay acute transformation.