The clinical course of a 66 year old male with CML who experienced several " relapse " BC of changing morphology, immunologic phenotype and cytogenetics is reported. The first BC was of lymphoblastic phenotype, the second of myeloid, the third again of lymphoblastic, and the fourth, terminal BC was not further characterized or treated. Whereas a phenotypic switch from lymphoblastic to myeloid has been documented twice, the sequence of myeloid followed by a lymphoblastic BC has, to our knowledge, not been reported so far. Lymphoblastic BC responded to a combination of vindesine/prednisone and 6-mercaptopurin. Myeloid BC was controlled by an AML-type induction regimen consisting of daunorubicin and cytosine arabinoside. This unusual case demonstrates that relapse BC is not necessarily of the same morphologic and phenotypic lineage as the preceding BC. Moreover, treatment should be adjusted to the predominant cell type in order to prolong survival.