To clarify the role of donor leukocyte infusion (DLI) in the treatment of leukemia relapsing after allo-BMT, data from 100 patients were collected from 46 facilities in Japan and analyzed with respect to the efficacy and adverse effects of donor leukocyte infusion. Complete remission was achieved in 11 of 12 (91%) patients with relapsed chronic myelogenous leukemia (CML) in chronic phase, three of 11 (27%) with CML in the acute phase, eight of 21 (38%) with acute myelogenous leukemia (AML), six of 23 (25%) with acute lymphoblastic leukemia (ALL) and five of 11 (45%) with myelodysplastic syndrome (MDS). The probability of remaining in CR at 3 years was 82% in CML patients in the chronic phase, but 0% in those with CML in the acute phase, 7% in those with AML, 0% with ALL and 33% with MDS. Acute GVHD (>/=2) developed in 31 of 89 (34%) patients with HLA-identical related donors and was fatal for seven (7%). Cytopenia developed in 21 of 94 (22%) with no associated fatalities. When the outcome of patients with CML in CP and MDS was analyzed, development of GVHD, cytopenia, or both, was associated with a higher GVL effect (15 of 16, 93%) than in those without adverse affects (one of 6, 17%). A leukocyte dose of 5 x 107/kg of recipient body weight appeared to be optimal as an initial dose of DLI. Given the relatively low incidence of acute GVHD and the similar GVL effect, DLI may be more beneficial to patients in Japan with recurrent leukemia than to those in Western countries. Bone Marrow Transplantation (2000) 26, 769-774.