A 50-year-old man had chronic myelogenous leukemia and entered a blast crisis that was both morphologically and histochemically lymphoid. The blasts contained terminal deoxyribonucleotidyl transferase and expressed lymphoblastic leukemia-associated antigen. He rapidly entered remission with vincristine sulfate and prednisone therapy. Nevertheless, his blasts displayed a marker generally considered unique to myeloid cells: they selectively bound the granulocyte chemotaxin N-formyl-Met-Leu-Phe. In addition, some cells contained granules resembling those of basophils or mast cells. Such mixed myeloid-lymphoid features in chronic myelogenous leukemia blast cells may reflect malignant transformation of a stem cell capable of both myeloid and lymphoid differentiation, or they may reflect the dedifferentiation as a feature of malignant change.