One hundred thirty patients with severe aplastic anemia were conditioned with cyclophosphamide for transplantation of marrow from HLA-identical siblings. The patients were selected for the present analysis according to the criterion of sustained marrow engraftment. Of the 130 patients, 97 are now alive between 1.4 and 11 years (median, 5) after transplantation. Twenty-nine of the thirty-three who died had either acute or chronic graft-versus-host disease (GVHD). Our analysis was directed at identifying factors predicting GVHD and survival after transplantation in patients. Our key findings were that moderately severe to severe acute GVHD had a strong adverse influence on survival; that a protective environment significantly reduced mortality, which corresponded in part to a reduction in and delayed onset of acute GVHD; that refractoriness to random-donor platelet infusions at transplantation adversely influenced survival, particularly among patients with acute GVHD; and that increasing age was associated with increased mortality. (N Engl J Med. 1983; 308:302–7.)