We report a patient with Philadelphia chromosome (Ph1)-positive chronic myelogenous leukemia (CML) which transformed into blast crisis bearing the immunophenotypic features similar to those of the myeloid/natural killer (NK) cell precursor leukemia we proposed previously. Using a CD45 blast gating method, the myeloperoxidase-negative blasts were positive for CD7, CD13, CD33, CD34, CD56, and HLA-DR, but no other lymphoid antigens. Southern blot analysis showed germ line T cell receptor beta and delta genes and immunoglobulin heavy and light chain genes. Although NK cell blastic transformation with Ph1 positive CML has been reported in a single patient, this is, to our knowledge, the first report of CML blast crisis of myeloid/NK cell precursor origin.