Leukemic cell DNA from patients with Philadelphia chromosome positive chronic granulocytic leukemia in the United Kingdom, Taiwan, and South Africa and of diverse ethnic origins all have identifiable molecular rearrangements of the breakpoint cluster region on chromosome 22 band q11 when screened with an appropriate DNA probe. This result reinforces the highly conserved nature of the molecular lesion in chronic granulocytic leukemia and its suitability as a diagnostic marker for the disease. Since the assay can be performed by sample referral on relatively small numbers of nondividing frozen or dead cells, it is ideally suited for large scale epidemiological and clinical studies, particularly in developing countries where karyotyping services are not readily available.