We took statistics of the 64 cases of chronic myelogenous leukemia admitted to our clinic during the past 16 years from the beginning of 1952 to the end of 1967 and studied them carefully with special reference to the criteria for early diagnosis of acute exacerbation and correlation between the acute exacerbation and the bone marrow culture of patients. As a result we obtained some new findings as follows. 1. Hematological and clinical criteria suggesting acute exacerbation prior to the appearance of hiatus leukemics in peripheral blood and/or bone marrowpictures are: (1) there is an increase of over 5% of myeloblasts in the peripheral blood and/or bone marrow: (2) an increase of over 20% of promyelocytes in the above mentioned pictures: (3) an increase of more than 5% of basophils in the above mentioned pictures: (4) a decrease of erythrocyte count below 3,000,000: (5) a decrease of platelet count below 100,000: (6) so called "dry tap" in the bone marrow: (7) as the clinical symptoms we find splenomegaly, a tendency to bleed, high grade fever, hepatomegaly, swelling of lymph nodes, general malaise, loss of appetite, and acute severe pain in the extremities.: and the growth pattern of the bone marrow tissue culture devised in our clinic approaches to the pattern of acute leukemia. In any case where three or more findings memtioned above are recognized, we can strongly suspect an acute exacerbation. 2. In the group treated with anti-leukemic drugs until the bone marrow culture showed 'hypoplastic pattern' the percentage of acute exacerbation proved to be 64.7% and the median survial time was 30.6 months, while in the group in which the treatment was shifted to maintenance therapy with minimum dosis of anti-leukemic drugs as soon as the bone marrow culture reached 'normal pattern', the percentage of acute exacerbation was 48.5% and the median survival time 37.6 months. These findings seem to suggest that chemotherapy of chronic myelogenous leukemia should be carried out tenderly with minimum dosis of anti-leukemic drugs and it should be maintained to the degree not so as to induce 'hypoplastic pattern' of bone marrow.