Few established cell lines derived from patients with chronic myelogenous leukemia have been reported. Cytogenetic examinations of two independently derived Philadelphia chromosome (Ph)-positive cell lines from a patient with chronic myelogenous leukemia were performed serially from their initiation in 1982 to the present. Subcultures of each of these lines maintained separately in two laboratories for over 2 years were compared for degree of divergence. The modal chromosome number declined substantially within the first few months and slowly thereafter. Despite hyperploidy, these lines have remained remarkably stable cytogenetically. For each line, the modal chromosome number is hypotetraploid, multiple copies of Ph are present and no normal chromosome #9 remains. Only a few marker chromosomes have arisen. Despite the multiple copies of Ph, a single bcr restriction pattern was seen, suggesting duplication of a single Ph, rather than independent translocation events. These lines should be very useful for in vitro studies of chronic myelogenous leukemia.