Abstract The clinical and morphologic features of nine patients who initially presented with blastic leukemia and the Philadelphia chromosome were studied. Corresponding features were evaluated at the time of diagnosis of blast crisis in 19 patients who had a previous history of chronic myeloid leukemia (CML). Although many of the presenting symptoms and signs were similar, infections, lymphadenopathy, tissue infiltration and central nervous system involvement were more common in patients who presented with blastic leukemia. Marked leukocytosis, basophilia and marrow hypercellularity were present in both groups. Although patients in both groups had morphologic patterns that resembled acute leukemia, cytology suggestive of acute lymphocytic leukemia was more frequent in patients who initially presented with blastic leukemia. Megakaryocyte, platelet and erythroid abnormalities were more frequent in patients with a prior history of CML. Although there were clinical and morphologic features in the patients who presented with blastic leukemia which suggested the diagnosis of CML in blast crisis, chromosome studies were necessary to identify some of these patients. In both groups of patients multiple therapeutic regimens were used. Complete remissions were obtained in two patients; both presented with blastic leukemia, had “lymphoblastic” morphology and were treated with chemotherapeutic agents generally used for the treatment of acute lymphocytic leukemia. It appears that morphology of the blast crisis may be important in choosing the treatment regimen.