The terminal phase of most patients with Ph1-positive chronic granulocytic leukemia (i.e., blast crisis) resembles acute leukemia. The clinical and hematologic features of blast crisis in 73 patients with chronic granulocytic leukemia have been reviewed. Two major morphological subgroups, lymphoblastic and myeloblastic, were identified. The lymphoblastic group in general had more profound thrombocytopenia and a greater number of blasts, while the myeloblastic group had more severa anemia. Extramedullary leukemia was documented in 27 patients. In 12 patients extramedullary leukemia preceded or occurred simultaneously with blast crisis in the bone marrow and peripheral blood. On the basis of this study we present hematologic criteria for the diagnosis of blast crisis and emphasize the importance of extramedullary leukemia in heralding the onset of blast crisis.