Fourteen children with high risk leukaemia received allogeneic bone marrow transplants from HLA-identical MLC-compatible sibling donors. All bone marrows were T cell depleted and a T cell addback was prepared from the donor's peripheral blood so that the mean total number of CD3+ cells given was 2.6 (1.0-4.1) x 10(5)/kg recipient body weight. This was administered as a short infusion prior to the bone marrow. The children were conditioned with 1440 cGy fractionated total body irradiation and cyclophosphamide 120 mg/kg and were not given cyclosporin A or methotrexate. All patients engrafted and none showed late graft rejection. Acute graft-versus-host disease (GVHD) developed in nine of 14 children and required treatment with steroids. Two children with grade IV GVHD and one with grade I acute GVHD who subsequently developed severe chronic GVHD died. There have been two relapses (both fatal) and one death from cytomegalovirus pneumonitis. Survival is currently 57% (8/14) with a mean follow-up of 548 days (range 384-810). A high incidence of GVHD which was fatal in three patients can occur despite infusion of low T cell numbers in the absence of post-graft immunosuppression.