OBJECTIVE: To characterize in detail the outcomes of HLA-identical sibling bone marrow transplantation for chronic lymphocytic leukemia (CLL) in patients younger than 60 years of age. DESIGN: Retrospective cohort study. SETTING: 30 centers for bone marrow transplantation worldwide, which reported data on outcome of HLA-identical sibling bone marrow transplantation for CLL to the European Group for Blood and Marrow Transplantation or the International Bone Marrow Transplant Registry between 1984 and 1992. PATIENTS: 54 patients diagnosed with CLL (median age, 41 years; range, 21 to 58 years). The median interval from diagnosis to transplantation was 37 months (range, 5 to 130 months). At the time of transplantation, 3 patients were at Rai stage 0; 10 were at stage 1; 10 were at stage 2; 7 were at stage 3; and 22 were at stage 4. INTERVENTION: Transplant regimens varied. Most patients received high-dose cyclophosphamide and total body irradiation, followed by infusion of bone marrow from an HLA-identical sibling. After transplantation, immune suppression with cyclosporine or methotrexate or both was generally used to prevent graft-versus-host disease. MEASUREMENTS: The primary outcome was survival. We also studied hematologic remission, defined as normalization of the leukocyte count, hemoglobin level, and platelet count, and absence of lymphadenopathy and splenomegaly. RESULTS: 38 patients (70%) achieved hematologic remission. Twenty-four (44%) remain alive a median of 27 months (range, 5 to 80 months) after transplantation. Three-year survival probability was 46% (95% CI, 32% to 60%). Three patients who received transplants at Rai stage 0 remain alive 21, 32, and 45 months after transplantation. Three-year survival probabilities were as follows: 68% (CI, 38% to 98%) in 10 patients who received transplants at Rai stage 1, 30% (CI, 2% to 58%) in 10 patients who received transplants at Rai stage 2, 57% (CI, 21% to 93%) in 7 patients who received transplants at Rai stage 3, and 34% (CI, 12% to 56%) in 22 patients who received transplants at Rai stage 4 CLL. Five patients (9%) died of progressive leukemia and 25 (46%) of treatment-related complications. CONCLUSIONS: Bone marrow transplants from HLA-identical siblings can result in hematologic remission and survival in persons with CLL, but it is uncertain how these results compare with those of conventional therapies.