A 39-year-old man with chronic myeloid leukemia in accelerated phase underwent allogeneic bone marrow transplantation (BMT). At 6 months after BMT, lymphocytosis (WBC count, 23,100/microL [23.1 x 10(9)/L]; 80% (0.80) large granular lymphocytes [LGLs]) occurred. The LGLs were CD3+CD4-CD8+, with clonally rearranged T-cell receptor gamma gene, and of donor origin, as shown by analysis of polymorphic microsatellite markers. Epstein-Barr virus was not present. The diagnosis, therefore, was consistent with T-cell large granular lymphocytic (T-LGL) leukemia. Corticosteroids controlled the LGL count, but progressive pancytopenia led to death 4 months later. Retrospective analysis showed that the T-LGL leukemia apparently had arisen as early as 3 months after BMT. The distinguishing features of this case included donor origin, neoplastic nature, and the aggressive fatal outcome.