Cytological and clinical characteristics of 25 patients with adult Ph1-positive acute leukemia were investigated. They were 2 cases with acute myelogenous leukemia (AML) and 23 cases with acute lymphoblastic leukemia (ALL). The prognosis of the patients with ALL, whose leukemia cells were positive for monoclonal antibodies against CD13 and/or CD33, was poorer than that of the patients with typical ALL. Additional chromosomal abnormalities were frequently detected on chromosome No. 2, 7, 8, 9 and 14. Both two patients with AML showed the additional chromosomal abnormalities on chromosome No. 8. Major- and minor-BCR rearrangements were analyzed in 14 patients with Ph1-positive acute leukemia. Neither major- or minor-BCR rearrangement was detected in one patients. Four patients showed major-BCR rearrangement and 9 patients showed minor-BCR rearrangement. By the chemotherapy including vincristine and prednisolone, 20 patients out of 25 got into complete remission. Nineteen patients, however, relapsed thereafter. Survival curves drawn by the method of Kaplan and Meier showed that 50 percent of the patients died within one year after diagnosis and that the prognosis of the patients with Ph1-positive acute leukemia was similar to that of the patients with chronic myelogenous leukemia in lymphoid blast crisis and worse than that of the all patients with ALL.