Chronic myelogenous leukemia (CML) is a relatively rare disease, with a number of features that make it especially suitable for observations on leukemogenesis in man. These include known etiologic agents, often a prolonged preclinical and active stage, cells with identifiable morphological and histochemical characteristics and the presence of the unique Ph' chromosome. The transition of CML to blast crisis is a catastrophic event; however, recent clinical and biochemical studies have raised important questions as to the nature and origin of blast cells in this disease. During the past 10 years, we have followed 113 cases olf CML throughout their course and results of observations on the clinical, hematological and other apsects of the disease are presented in this communication.