Background: Both immunosuppressive therapy and bone marrow transplantation are accepted treatments for patients with aplastic anemia. Choosing one of these therapies for a given patient depends not only on donor availability but also on such factors as patient age. Objective: io compare survival rates and long-term complications after bone marrow transplantation or immunosuppressive therapy in patients with acquired aplastic anemia and to identify prognostic factors associated with improved survival. Design: Center-based, retrospective analysis. Setting: Referral center for patients with aplastic anemia. Patients: 395 patients with acquired aplastic anemia. Intervention: Bone marrow transplant from an HLA-identical, related donor or immunosuppressive therapy. Measurements: Kaplan-Meier survival curves, results of log-rank tests, and cumulative incidence curves. Results: Of 168 bone marrow transplant recipients, 89% had sustained engraftment. Forty-six patients developed grade II to IV acute graft-versus-host disease, and 68 developed chronic graft-versus-host disease that required therapy. Of 227 patients who received immunosuppressive therapy, 44% achieved a complete, partial, or minimal response. Fifty-four percent died or had no response to therapy. Actuarial survival at 15 years was 69% for bone marrow transplant recipients and 38% for patients receiving immunosuppressive therapy (P < 0.001). Improved survival was associated with having bone marrow transplantation as primary therapy, being younger, having no transfusion before transplantation, and having a higher absolute neutrophil count. Disease duration, year of therapy, sex, refractoriness to platelet transfusions, and previous treatment with androgens or corticosteroids did not significantly affect survival. Conclusions: Data from this center suggest that bone marrow transplantation may be preferred for younger patients with acquired aplastic anemia who have matched, related donors. Long-term survival is excellent for patients who respond to either form of therapy.