We report a patient with myelodysplastic syndrome (MDS) overt leukemia who developed an acute myeloid leukemia (AML) in donor cells shortly after bone marrow transplantation (BMT) from his HLA-matched sibling. Molecular analysis using microsatellite repeats by polymerase chain reaction proved the new leukemia to be of donor cell origin. The patient received chemotherapy with idarubicin and ara-C, but he died due to mucormycosis.