The immune reactivity of allogeneic lymphocytes plays a major role in control of leukemia after bone marrow transplantation. We studies the efficacy of donor leukocyte transfusion (DLT) on acute and chronic leukemia in relapse after bone marrow transplantation in Japan. Sixty nine patients with chronic myelocytic leukemia (N = 17), acute lymphoblastic leukemia (N = 25), acute myelocytic leukemia (N = 26), myelodysplastic syndrome (N = 5), non-Hodgkin lymphoma (N = 2) and rhabdomyosarcoma (N = 1) were treated with transfusions of donor lymphocytes. Therapeutic effects were induced by donor leukocyte transfusion in 20 patients (29%) including 3 patients out of 4 (75%) with CML in cytogenetic and chronic phase relapse, 4 out of 5 (80%) patients with myelodysplastic syndrome, 3 out of 13 (23%) patients with CML in transformed phase, 5 out of 25 (20%) patients with acute myelocytic leukemia, and 4 out of 20 (20%) patients with acute lymphoblasic leukemia. Twenty two patients (30%) developed acute GVHD (> or = 2) and 6 out of 73 (8.2%) patients developed fatal GVHD after donor leukocyte transfusion. Patients relapsed within 6 months after marrow transplantation had a probability of having severe acute GVHD (> or = 2) after DLT. Fourteen out of 24 (58%) patients with GVL response were re-relapsed thereafter. Minimal dose of donor leukocytes infused in successfully treated 9 patients without cytoreductive therapy was 2 x 10(7)/kg in total and minimal dose of that in 6 patients with fatal GVHD was 7 x 10(7)/kg in total. The anti-leukemia effect of donor leukocyte transfusion was strongest against CML in cytogenetic and chronic phase and induce a durable complete remission.