The breakpoint localization was analyzed in 61 patients with Philadelphia chromosome (Ph) positive chronic myelogenous leukemia to compare the breakpoint localization and clinical course. All patients were treated with interferon alfa (IFN alpha) or IFN alpha plus IFN gamma at the time of the study. Thirty-three of the patients had been pretreated with other cytostatic drugs. Sixty-nine per cent of the breakpoints were located in the 5' region of the major breakpoint cluster region (M-bcr), 29% in the 3' part. There was no significant difference between these two groups with respect to response to IFN(s), clinical course or conversion to blast crisis, nor survival.