Chronic myelogenous leukemia (CML) is characterized by two distinct phases--chronic and acute. Features of the chronic phase include proliferation and accumulation of mature myeloid cells and their progenitors; differentiation is seemingly intact. In contrast, the acute phase is characterized by impaired differentiation. Acute phase is heterogeneous--granulocytes, lymphocytes, megakaryocytes, and erythroid cells are involved singly or in combination. The t(9;22) translocation, which results in the Ph1 chromosome, is the hallmark of chronic phase CML. Transition to acute phase is often accompanied by additional chromosome abnormalities. Here we suggest that these additional abnormalities determine the phenotype of acute phase CML.