Reciprocal translocations, in addition to that of the Ph chromosome, though rare, have been reported in chronic myelogenous leukemia (CML). We describe here a case of Ph-positive CML with a new translocation, t (11;21) (q13;q22), and missing Y, which were present both during transformation to the blastic crisis and in the subsequent reversion to the chronic phase. The possible significance of this abnormality is discussed.