A new abnormality of chromosome 17, distinct from i(17q), was observed in bone marrow cells of two patients with CML during a blast crisis. The marker was identified as a translocation of the short arm of chromosome 5 onto the long arm of chromosome 17, i.e., t(5p;17q). In one case, the marker was clearly dicentric. The clinical history of these two patients showed some similarities. A slightly different marker has previously been observed in a cell line derived from a solid tumor.