Cytogenetic and molecular characteristics of chronic myeloid leukemia in blast crisis and Ph-positive acute leukemia are compared. The main differences relate to the presence of Ph-negative metaphases at diagnosis and the disappearance of Ph in complete remission in acute leukemia, and the localization of the chromosome breakpoints in the BCR gene, in the bcr segment in chronic leukemia and in the first intron of the BCR gene in 50% of acute leukemias. The profiles of these abnormalities, as well as the types of additional chromosome changes, are not sufficient to distinguish between the two disorders in every patient. The distinction between these two entities, which is possible in the majority of patients, will be improved by results of experimental work currently in progress in many laboratories.