Data were obtained from 46 patients with severe aplastic anemia (SAA) who received bone marrow transplants (BMT) from donors other than genotypically HLA-identical siblings. The data were collected in the SAA Registry of the European Bone Marrow Transplant Group. The donors were non-HLA-identical siblings in six cases, parents in 28 cases, a son in one case and unrelated individuals in 11 cases. Fifteen donor-recipient pairs were HLA-A, -B and -DR identical and mutually non-reactive in mixed lymphocyte culture; nine were mismatched at one locus, 17 were mismatched at two or more loci and in five cases data were not available for D/DR determinants. Actuarial survival was predicted by the degree of mismatch. It was 45% for phenotypically HLA-identical grafts, 25% for grafts mismatched at one locus and 11% for graft mismatched at more than one locus. Whether the graft was derived from a family member or an unrelated donor seemed to be less important and results were comparable. Age, patient sex and year of transplant had no significant influence on survival. The use of cyclosporine (CSA) for graft-versus-host disease (GVHD) prophylaxis (n = 21, survival 34%) appeared superior to both methotrexate (n = 9, survival 11%) and to CSA with T cell depletion of donor marrow (n = 13, survival 14%). The causes of death were rejection (n = 15), GVHD (n = 13), pneumonitis (n = 5) and infection (n = 1). Twelve patients are alive at 16-84 months post-BMT.(ABSTRACT TRUNCATED AT 250 WORDS)