Fifty-seven patients undergoing allogeneic marrow transplantation had persistent poor graft function after transplant and received a second marrow infusion from the original donor. Eight donors were HLA-non-identical family members and 49 were HLA-identical siblings. Poor function of the initial graft occurred in the absence of demonstrable rejection or persistent malignancy. Preparative reconditioning was not given before the second marrow infusion. Thirty-four of the 57 patients survived more than 1 month after the boost and were evaluated for haematopoietic recovery. All 34 subsequently achieved a neutrophil count of more than 500 X 10(6)/l and 20 became independent of platelet transfusions. Forty-nine (86%) of the 57 patients demonstrated acute graft-versus-host disease (GVHD). In 24 of the 49 patients GVHD increased in severity (10 patients) or first appeared (14 patients) after the boost. Chronic GVHD developed in all of the 20 patients who survived more than 150 days after the second infusion. Currently, 10 patients survive (median follow-up of more than 5 years) and all have normal haematologic function. Although haematologic reconstitution and increased GVHD occurred in some patients receiving second marrow infusions, the relation of these outcomes to the marrow boost is unclear in the absence of a control group.