Seventy-three consecutive patients with severe aplastic anemia were treated by marrow grafts from normal, HLA-identical siblings, and 68 lived long enough to demonstrate engraftment. In 21 patients the garft was rejected, and 19 of these patients died. This analysis, using a binary logistic regression model, was aimed at identifying factors that predicted marrow-graft rejection. Of the 24 factors entered into the analysis, only two strongly correlated with graft rejection: a positive relative response index in mixed leukocyte culture indicating sensitization of patient against donor (P less than 0.01); and a low number of marrow cells ( less than 3 X 10(8) cells per kilogram) used for transplantation (P less than 0.01). These findings suggest that more powerful immunosuppressive conditioning regimens should be used in patients who are sensitized, and that the greatest possible amount of donor marrow, perhaps supplemented by stem cells derived from the peripheral blood, should be obtained.