Although T-prolymphocytic leukemia (T-PLL) is characterized by organ infiltration, small-intestinal involvement is rare. We performed an unrelated allogeneic bone marrow transplantation in a patient with T-PLL who had multiple lymphomatous polyposis of the small intestine refractory to combination chemotherapy (cyclophosphamide, vincristine, and prednisolone [COP] and fludarabine plus cyclophosphamide). The patient developed no graft-versus-host disease (GVHD) and remains in complete remission 16 months after the transplantation. T-PLL is usually refractory to chemotherapy and is a T-cell malignancy with poor prognosis. There have been several reports on allogeneic hematopoietic stem-cell transplantation (allo-HSCT) for T-PLL, but none on allo-HSCT for T-PLL patients with intestinal involvement. It is suggested that allo-HSCT may improve the prognosis in patients with T-PLL involving the small intestine.