Do some patients benefit from an unrelated donor(URD) transplant because of a stronger graft-versus-leukemia(GVL) effect? We analyzed 4099 patients with acute myeloid leukemia(AML), acute lym-phoblastic leukemia(ALL), and chronic myeloid leukemia(CML) undergoing a myeloablative allogeneic hematopoietic cell transplantation(HCT) from an URD(8/8 human leukocyte antigen [HLA]-matched, n = 941) or HLA-identical sibling donor(n = 3158) between 1995 and 2004 reported to the CIBMTR. In the Cox regression model, acute and chronic GVHD were added as time-dependent variables. In multivariate analysis, URD transplant recipients had a higher risk for transplantation-related mortality(TRM; relative risk [RR], 2.76; P < .001) and relapse(RR, 1.50; P < .002) in patients with AML, but not ALL or CML. Chronic GVHD was associated with a lower relapse risk in all diagnoses. Leukemia-free survival(LFS) was decreased in patients with AML without acute GVHD receiving a URD transplant(RR, 2.02; P < .001) but was comparable to those receiving HLA-identical sibling transplants in patients with ALL and CML. In patients without GVHD, multivariate analysis showed similar risk of relapse but decreased LFS for URD transplants for all 3 diagnoses. In conclusion, risk of relapse was the same(ALL, CML) or worse(AML) in URD transplant recipients compared with HLA-identical sibling transplant recipients, suggesting a similar GVL effect.