Six cases of chronic myelogenous leukemia (CML) with coexistence of cells with a Philadelphia (Ph) translocation and a normal karyotype are described. Molecular analyses were performed in 3 of the 6 cases at two phases with different cytogenetic findings. Among the 6 cases, 4 cases were late appearance of a Ph chromosome. Two of the 3 patients analyzed had major breakpoint cluster region (Mbcr) rearrangements; aberrant Mbcr fragments were observed whether the cells had a Ph translocation or not. The remaining one did not show any rearrangements within the Mbcr DNA sequences.