The occurrence of acute bilineage leukemia is thought to be the malignant transformation of a myeloid or lymphoid leukemic progenitor with the potential to differentiate into the other lineages; however, the mechanisms of this lineage switch are not well understood. Here, we show the extremely rare case of adult Philadelphia chromosome positive acute bilineage leukemia, characterized by T-cell acute lymphoblastic leukemia, CD7+CD5+CD14−, and acute myelomonocytic leukemia, CD7−CD5−CD14+. Chromosome analysis showed 46,XY,del(7)(p11.2),t(9;22)(q34;q11.2) in all metaphase and leukemic cells expressed a minor BCR/ABL chimeric gene. When the CD5+CD14− and CD5−CD14+ cells were sorted, a fusion gene of BCR/ABL and a same clonal rearranged band of a T-cell receptor (TCR) gene were detected in both populations. Nucleotide sequencing of the TCRg gene revealed the clonal rearrangement of the V8-JGT2 complex in both populations. Over-expression of PU.1, which plays a fundamental role in myelomonocyte development was found in the sorted CD34+CD7+ and CD5−CD14+, but not CD5+CD14− cells. These results suggest that leukemic progenitor cells in the T-lineage with del(7),t(9;22) chromosome have the potential to differentiate into myeloid lineage and enforced PU.1 expression may contribute in part of this phenomenon. Studies of bilineage leukemia will be important for the understanding of lineage commitment and switch in hematopoietic cells.