The specific genetic events that distinguish the blast crisis from the chronic phase cells of chronic myelogenous leukemia (CML) are unknown. The most common karyotypic change that occurs as CML evolves from chronic phase to blast crisis is the development of multiple Philadelphia (Ph1) chromosomes, each of which is presumably harboring a translocated c-abl oncogene. We describe here a patient with CML who presented in lymphoid blast crisis with three Ph1 chromosomes/metaphase associated with an amplified, rearranged c-abl oncogene fragment and high levels of the aberrant 8-kilobase bcr-abl transcript. This rearranged c-abl fragment was amplified to a similar degree in both the patient's blast crisis cells and in his terminally differentiated granulocytes, but the level of the aberrant CML-specific bcr-abl transcript was some eight- to 16-fold higher in the blast crisis cells v the granulocytes. This analysis indicates that genomic amplification of a translocated c-abl oncogene, although perhaps important in the evolution of CML, nevertheless cannot, by itself, be the sole genetic event giving rise to blast crisis.