Although a breakpoint in the minor breakpoint cluster region (m-bcr) of the BCR gene is observed in about two-thirds of patients with Philadelphia chromosome-positive acute lymphoblastic leukemia, this type of genomic rearrangement occurs very rarely in chronic myeloid leukemia (CML). We describe here the eighth case of m-bcr CML, and delineate unique clinical characteristics found in common to the 7 cases reported previously. Monocytosis with a low neutrophil/monocyte ratio resembling chronic myelomonocytic leukemia was the most striking feature of m-bcr CML. Splenomegaly and basophilia were not conspicuous in chronic phase. A high percentage of immature granulocytes and low neutrophil alkaline phosphatase score were the findings in common with classical CML. Lymphoid and myeloid blast changes have been observed at and shortly after presentation so far. We found a hybrid type of blast crisis in the course of m-bcr CML.Thus, m-bcr CML may be a definite subtype of CML, exhibiting distinct clinical characteristics. The presence of fusion product of m-bcr mRNA in an earlier myeloid cell may involve monocytic lineage in addition to myeloproliferative defects.