The blast crisis in chronic myelogenous leukemia (CML) is related to the evolution of a Philadelphia chromosome (Ph)-positive clone. Secondary chromosomal abnormalities accompanied by t(9;22) are found in 70-80% of blast crises. Here we describe a patient with Ph-positive CML, who developed Ph-negative acute lymphoblastic leukemia (ALL). A 52-year-old man was diagnosed with CML with the Ph chromosome in the chronic phase. He achieved a partial cytogenetic response after 4 months of imatinib mesylate therapy. After 8 months, common ALL occurred. At that time his karyotype was normal and the Ph chromosome was not noted.