Allogeneic bone marrow transplantation (BMT) offers the only curative therapy for chronic myelogenous leukemia. We compared prospectively collected results of 2464 unrelated donor (URD) transplantations with 450 HLA-identical, matched sibling donor (MSD) transplantations performed at collaborating National Marrow Donor Program institutions. A total of 63% of URDs were matched at HLA-A, -B, and at -DRB1 alleles; all MSDs were geno-typically identical at major histocompatibility loci. URD recipients were younger (median 36 vs 39, P = .001) than MSDs and underwent BMT later after diagnosis (median 17 [0-325 months] vs 7 [1-118 months], P = .001) and less often in chronic phase (CP) (67% vs 82%, P = .001). Multivariate analysis demonstrated a significantly increased risk of graft failure and acute graft versus host disease after URD BMT. The risk of hematologic relapse was low after either matched URD or MSD transplantations. We observed significantly though modestly poorer survival and disease-free survival (DFS) after URD transplantations. However, for those undergoing transplantation during CP within 1 year from diagnosis, 5-year DFS was similar or only slightly inferior after matched URD versus MSD transplantation (age < 30: URD 61% ± 8% vs MSD 68% ± 15%, P = .18; 30-40: URD 57% ± 9% vs MSD 67% ± 10%, P = .05; > 40: URD 46% ± 9% vs MSD 57% ± 9%, P = .02). Delay from diagnosis to BMT in CP patients led to substantially poorer 5-year DFS after matched URD than MSD BMT (CP 1-2 years: URD 39% ± 6% vs MSD 63% ± 12%; beyond 2 years: URD 33% ± 7% vs MSD 50% ± 20%). Outcome of matched URD BMT for early CP chronic myelogenous leukemia yields survival and DFS approaching that of MSD transplantation. However, delay may compromise URD outcomes to a greater extent. Improvements in URD and MSD transplantation are still needed, and results of newer, nontransplantation therapies should be evaluated against the established curative potential of URD and MSD marrow transplantation.