Chronic granulocytic leukemia (CGL) terminates in a disease similar to acute leukemia. Except for a study indicating an increased response rate to the drugs, vincristine and prednisone, therapy of this terminal phase has been universally disappointing. We have studied the bone marrows and clinical courses of 67 patients in the terminal phase of CGL to discern if any parameters were associated with an increased response rate or survival. The results of this study indicate that patients who have a lymphoblastic morphology or hypodiploid cytogenetics in the terminal phase respond better to treatment with the combination of vincristine and prednisone than those with myeloblastic morphology or hyperdiploid cytogenetics. Response rate and survival are significantly increased in those with lymphoblastic morphology. Recognition of the heterogeneity of the terminal phase of CGL may dictate specific therapeutic modalities.