T-cell lymphoproliferative disorders are uncommon occurrences after bone marrow transplantation (BMT). We describe 2 patients in whom a monoclonal T-cell large granular lymphocytosis (T-LGL) developed after allogeneic BMT for B-cell lymphoproliferative disorders. Both patients showed a persistent expansion of CD3+, CD8+, and CD57+ large granular lymphocytes of donor origin with clonally rearranged T-cell receptor gamma genes and no evidence of Epstein-Barr virus-related infection. The manifestations were consistent with T-LGL leukemia as defined by the World Health Organization criteria. In both patients, graft-vs-host disease developed, and 1 had recurrent episodes of cytomegalovirus viremia. The other patient had received a graft from a hepatitis C antibody-positive donor without developing any signs of hepatitis C infection. Both patients remain in complete remission from their B-cell lymphoproliferative disorders and do not have symptoms related to T-LGL leukemia. These data show that T-LGL leukemia should be included as one of the types of posttransplantation lymphoproliferative disorders that can occur after allogeneic BMT for B-cell neoplasms.