Comprehensive data were reported to the International Bone Marrow Transplant Registry by 24 worldwide teams regarding 144 patients with severe aplastic anemia who were treated with HLA-identical, allogeneic bone marrow transplantation between 1975 and 1978. One hundred fourteen patients received one transplant and 30 received two. Sustained engraftment of donor marrow occurred on the first transplant attempt in 76% of the patients whose engraftment status could be evaluated. Moderate, severe, or lethal graft-vs-host disease (GVHD) occurred in 52% of the engrafted patients. The one-year survival rate for all patients was 44% (64/144). Among the one-year survivors, the primary disease was apparently cured in 90% and improved in 10%. Thus far, four patients died one to five years after transplant. Of the 60 patients currently alive, ten have chronic GVHD and the remainder are in good health. Multivariate analyses of the data disclosed a number of pretransplant factors associated with engraftment, GVHD, and one-year survival. The most important finding was that the use of male donors was preferable to female donors because of a higher rate of engraftment, less severe GVHD, and a higher one-year survival rate.