A break in chromosome 22 within the major breakpoint cluster region (M-bcr) is a characteristic of Philadelphia chromosome-positive CML. We have determined the zone of the breakpoint in 80 chronic myelogenous leukemia (CML) patients and have confirmed our previous observation that a relationship does exist between the subregion of the breakpoint within the M-bcr and the average length of the chronic phase of the disease. Patients with a 3' breakpoint have a statistically shorter chronic phase (25 months) than patients with a 5' break (55 months). Thus, a molecular analysis of the M-bcr may provide a prognostically useful indicator of the probable length of the chronic phase, although the underlying mechanism of blast transformation, and the role (if any) of the hybrid phl-abl mRNA, is still unclear.