One hundred and seventy-five consecutive patients with severe aplastic anaemia were given high-dose cyclophosphamide followed by marrow grafts from healthy, HLA-identical family members, and 168 lived long enough to show engraftment. In 38 patients the graft was rejected and 29 of these died. This analysis, using a binary logistic regression model, was aimed at identifying factors that predicted marrow graft rejection. Five factors correlated with graft rejection: (1) previous blood transfusion; (2) a positive relative response in mixed leucocyte culture indicating sensitization of patient against donor; (3) a low number of marrow cells used for transplantation; (4) marrow grafts from male donors; and (5) lack of infusion of viable donor buffy coat cells in addition to the marrow for transfused patients. The findings confirm the importance of transplanting early before transfusion and indicate that the greatest possible amount of donor marrow (supplemented by stem cells/lymphoid cells derived from the peripheral blood) should be obtained.