A 24-year-old man with Philadelphia-chromosome (Ph)-negative chronic myelocytic leukemia (CML) developed lymphoid blast crisis. In the chronic phase, karyotype was normal and the clinical and hematological features were indistinguishable from those of Ph-positive CML. Rearrangement of the breakpoint cluster region (bcr) was observed. In the blast phase, blast cells showed early B-cell phenotype (CALLA +, Ia +, TdT+) with a rearranged immunoglobulin heavy-chain gene joining region (JH). By using an immuno-blotting method and antiphosphotyrosine sera, P210bcr-ab1 protein was detected. The patient responded well to vincristine and prednisolone (VP) therapy. These findings support the concept that Ph-negative bcr + CML can behave in a very similar fashion to Ph-positive CML, not only in the clinical features of the chronic phase but also in the manner of the blast crisis.