A 58-yr-old male patient presented in the blastic phase of chronic myelogenous leukemia (CML). Cytogenetic studies revealed a 45 XO Ph1 chromosome pattern in bone marrow cells during a short remission and again in the blastic phase of the disease. The patient expired 8 mo following diagnosis. The blastic phase of CML can stimulate acute myelogenous leukemia (AML) clinically and hematologically; CML can be differentiated by the presence of the Ph1 chromosome and the stigmata of CML. Absence of the Y chromosome from the bone marrow in CML is a recently described finding. Previous reports indicating the prevention of the blastic phase in patients with this karyotype could not be confirmed by our or other recently reported cases.