Twenty-five cases of Ph1-positive acute leukemia (AL) are described, 13 presenting an acute lymphocytic leukemia (ALL) and 12 an acute non-lymphocytic leukemia (ANLL). In 12 cases coarse, pink, peroxidase-negative cytoplasmic granules were found in the leukemic cells. These granules have not been described in Ph1-negative AL and their presence appears to be pathognomonic of Ph1-positive acute leukemia. The leukemias of three patients consisted of both lymphoid and myeloid clones while the cells of two patients had lymphoid and myeloid markers simultaneously present in the same cells. Cytogenetic studies were useful for monitoring response and some patients clearly acquired a Ph1-negative status during clinical remission. The disease appears to be more resistant to chemotherapy than Ph1-negative acute leukemia. While similar to chronic myelocytic leukemia (CML) in the Ph1 translocation, Ph1 AL differed from it both in age at presentation and response to therapy.