A 34-year-old male with acute nonlymphocytic leukemia (ANLL) and a t(6;9)(p23;q34) is described, in whom loss of the Y chromosome had occurred in the t(6;9) clone. The leukemic blasts were relatively undifferentiated and there was no increase in marrow basophils. The unusual clonal evolution and hematologic features in this patient are compared to 27 previously documented cases with this translocation.