Allogeneic hematopoietic stem cell transplantation (allo-HSCT) following myeloablative conditioning represents the treatment of choice for patients with chemotherapy-resistant leukemia. We describe a 49-year-old man with advanced, refractory acute myelogenous leukemia (AML) that was treated successfully by allogeneic bone marrow transplantation from a sibling donor with HLA mismatched at 1 locus. However, the patient developed a quiescent form of chronic graft-versus-host disease (GVHD) 7 years after transplantation, requiring long-term immunosuppressive therapy. AML relapse was documented 11 years after transplantation. Subsequently, Epstein-Barr virus (EBV)-associated posttransplantation lymphoproliferative disorder (PTLD) was also diagnosed. Immune reconstitution after allo-HSCT might have been impaired by the persistent chronic GVHD and the prolonged administration of immunosuppressive agents. As a result, immune surveillance against remaining quiescent leukemic cells as well as viral infection may have been defective, leading to the relapse of leukemia and EBV-associated PTLD.