From March 1975 until May 1980 twelve patients with severe aplastic anemia were grafted with bone marrow from HLA-identical siblings by the Munich Cooperative Group for Bone Marrow Transplantation. Six patients are alive between 10 months and more than 5 years after grafting with normal blood values and marrow. One patient is treated as an out patient for chronic localized graft-versus-host disease (GvHD), five patients are well and without treatment. Six patients have died, one patient with a cerebral hemorrhage the day before transplantation, three patients following rejection of grafts 32, 40 and 55 days after grafting, one patient with severe GvHD 85 days after grafting and one patient, probably with interstitial pneumonia, following cerebral hemorrhage. Three of 6 patients who were conditioned with Cyclophosphamide (CY) only died following rejection of the graft. Two adults who were conditioned with CY and "total lymphoid irradiation" and three children, who wer given unirradiated leukocyte concentrates from the marrow donor after grafting, did not reject their grafts. The results of the Munich-Cooperative Group for Bone Marrow Transplantation are comparable to those of large, specialized centers for bone marrow transplantation, they indicate possibilities of cure of severe aplastic anemia by marrow grafts from HLA-identical siblings. They confirm that better results are obtained with earlier transplantation in the course of the disease.