Detailed chromosomal analyses of eight patients with an acute blastic crisis (ABC) of chronic myelocytic leukemia (CML) who had two (diplo) or three (triplo) Philadelphia (Ph1) chromosomes showed all patients have marked aneuploidy. Seven were hyperdiploid and one hypodiploid. The mechanism involved in the chromosomal abnormalities, other than the Ph1chromosome was thought due to nondisjunction in mitosis. The most simple explanation for the nondisjunction would be that it occurs more frequently in cells which are already chromosomally imbalanced.