Allogeneic bone marrow transplantation (BMT) in humans is hampered mainly by graft-versus-host disease (GVHD). Ex vivo T-cell depletion of the marrow graft has decreased the incidence and severity of GVHD, but has resulted in a higher incidence of graft failure and of relapse of the disease. In order to find an optimal T-cell number that avoids the extreme risks on both sides, we performed BMTs with a fixed low number of T cells. Thirty-one patients received marrow grafts, containing 1 x 10(5) T cells per kilogram body weight, from their HLA-identical sibs. All patients, except one, received cyclosporin A. Engraftment of donor marrow cells occurred in all patients and (late) graft rejections are not observed to date. Eighteen of 30 (60%) evaluable patients had acute GVHD, grade I (10 patients) or grade II (8 patients), limited to the skin in all patients. Chronic GVHD, also limited to the skin, was found in 9 of 27 (33%) evaluable patients. Incidence but not severity of GVHD in our study seems similar to that observed in non-T-cell depleted marrow grafting. Relapse was observed in 1 of 13 leukemic patients transplanted in first (or second) remission or first chronic phase with a follow-up of at least 6 months. These results suggest that with a fixed low number of T cells severe GVHD and failure to engraft can be avoided. More patients and longer follow-up are necessary for conclusions regarding relapse rate and late graft failure.