In order to search for the factors prolonging life span of chronic myelogenous leukemia(CML), the survey of long-term survivors in Japan was attempted for the first time. CML patients surviving over 7 years from the initial diagnosis were surveyed by sending inquiring cards to 1,197 hospitals with more than 200 beds and the following results were obtained: 1) Seventy cases of long term-survivors in CML over 7 years, were collected by the end of March 1977, when 29 cases of them had been alive. The longest one survived 21.3 years after the initial diagnosis. 2) The annual incidence has tended to be increased since around 1963. 3) As for host factors, i) No difference in sex; ii) Age ranged from 14 to 70 years old with the highest incidence of 30-34, iii) Included were 11 cases of atomic-bomb survivors; about one half might be diagnosed at the early stage through regular health check. 4) As for tumor factors, leuko-, erythro- and thrombocyte counts and myeloblasts plus promyelocytes percentages in peripheral blood and bone marrow at initial diagnosis did not significantly differ between long-term survivors and randomlychosen control group died of CML within 7 years. Only splenomegaly at the diagnosis was significantly less in long-term survivors than that in controls. All 23 cases with chromosome analyzed were Ph(1) positive. 5) As for drugs, out of 27 cases, of whom all drugs given in the chronic phase were precisely recorded, 9 were treated with busulfan alone throughout course and 5 with busulfan and 6-MP alternately. 6-MP was given at least once in the chronic phase to 10 cases and adrenocorticosteroid to 3 cases. Eleven of 32 cases survived over 10 years without treatment over 5 years, suggesting an unusually-high sensitivity of leukemic cells in these cases to antileukemic agents. 7) Of 70 cases, 41 had already died by the time of the present survey. Thirty of 41 (79.2%) had died of blastic crisis. Median survival after blast crisis of these 30 cases was only 3.7 months. 8) In conclusion, specific factors frequently observed in long-term survivors in CML are: i) low grade of splenomegaly at the diagnosis, ii) high sensitivity of leukemic cells to antichemotherapeutics including busulfan, and iii) either an alternate use of busulfan and 6-MP or steroid on the CML patients in chronic phase. Evaluation of the effect of 6-MP or steroid on CML patients in chronic phase for prolonging life span should be comfirmed in future.