Therapy-related myelodysplastic syndrome in patients with acute promyelocytic leukemia is a rare event and the prognosis is poor. Allogeneic bone marrow transplantation is recently being reported as an effective treatment. We present a young patient with acute promyelocytic leukemia who developed myelodysplastic syndrome 52 months after complete remission. She underwent allogeneic peripheral blood stem cell transplantation but relapsed with biphenotypic leukemia after five months. To our knowledge, this is the first case to relapse with acute biphenotypic leukemia after allogeneic peripheral stern cell transplantation for therapy-related myelodysplastic syndrome following acute promyelocytic leukemia. © Turkish Society of Hematology.