In chronic myelogenous leukemia (CML) and in a percentage of childhood and adult acute lymphoblastic leukemia (ALL) the Philadelphia (Ph′) chromosome is present in the leukemic cells of patients. This chromosome is the result of a reciprocal translocation between chromosomes 9 and 22. In CML the break on chromosome 22 occurs within the major breakpoint cluster region (Mbcr) of the bcr gene. In this study, we report on the examination of DNAs from nine Ph′-chromosome positive ALL patients for rearrangements within the bcr gene using Southern blot analysis. Of nine patients having a karyotypically identifiable Ph′-chromosome, only five exhibited rearrangements of the bcr gene. This could indicate that in ALL, chromosome 22 sequences other than the bcr gene are involved in the Ph′-translocation. Within the group of Ph′-positive ALL patients having a bcr gene breakpoint, a correlation appears to exist between the age of the patient and the location of the breakpoint within the gene: all or the vast majority of pediatric patients analyzed to date do not have a Mbcr breakpoint as found in CML and in adult ALL.