Background and Methods: In contrast to autologous bone marrow transplants for hematologic cancers, allogeneic transplants contain no tumor cells that might cause a relapse. We report the results of such allogeneic bone marrow transplantation using HLA-compatible sibling donors in 90 patients with multiple myeloma performed in 26 European centers between 1983 and 1989. Results: At the time of the most recent follow-up, 79 months after the start of the study, 47 patients were alive and 43 were dead. The rate of complete remission after bone marrow transplantation was 43 percent for all patients and 58 percent for the patients who had engraftment. The actuarial survival at 76 months was 40 percent. The median duration of relapse-free survival among patients who were in complete remission after bone marrow transplantation was 48 months. The stage of the disease at diagnosis and the number of treatment regimens tried before bone marrow transplantation were predictive of the likelihood of complete remission after engraftment. There were trends toward longer survival among patients who were responsive to treatment before bone marrow transplantation, patients with Stage I disease at diagnosis, and patients who had received only first-line treatment before transplantation, as compared with those who were not responsive, those with Stage II or III disease at diagnosis, and those who had received three or more lines of treatment, but the differences in these factors were not statistically significant. Two post-transplantation factors predicted better long-term survival: complete remission after engraftment and grade I graft-versus-host disease, rather than grade II, III, or IV. Conclusions: Allogeneic bone marrow transplantation with the use of HLA-matched sibling donors appears to be a promising method of treatment for some patients with multiple myeloma. (N Engl J Med 1991;325:1267-73.)