Several studies show that allogeneic peripheral blood stem cells (PBSCs) engraft more rapidly than bone marrow (BM). However, the data are inconsistent with regard to acute and chronic graft-versus-host disease (GVHD), relapse, transplant-related mortality (TRM), and leukemia-free survival (LFS). Between January 1994 and December 2000, 3,465 adult patients (older than 15 years of age) were reported to the European Group for Blood and Marrow Transplantation Registry from 224 centers. Among acute myeloid leukemia (AML) patients, 1,537 patients received BM and 757 patients received PBSC. In acute lymphoblastic leukemia (ALL) patients, the corresponding figures were 826 versus 345 patients who were analyzed for engraftment, GVHD, TRM, relapse, LFS, and survival. In multivariate analysis, the recovery of neutrophils and platelets was faster with PBSC than with BM (P <.0001). Chronic GVHD was associated with PBSC in patients with AML (relative risk RR, 2.11; 95% confidence interval, 1.66 to 2.7; P <.0001) and ALL (RR, 1.56; 95% confidence interval, 1.09 to 2.27; P =.02). PBSC versus BM in patients with AML or ALL was not significantly associated with acute GVHD, TRM, relapse, survival, or LFS. In multivariate analysis of patients with AML, factors significantly associated with improved LFS included first remission at transplant (P <.0001), promyelocytic leukemia (M3) versus other French-American-British types (P <.0001), and donor age below median 37 years (P =.02). In patients with ALL, first remission (P <.0001) and methotrexate included in the immunosuppressive regimen (P =.001) were associated with improved LFS. Allogeneic PBSC results in faster neutrophil and platelet engraftment and a higher incidence of chronic GVHD than BM. However, acute GVHD, TRM, relapse, survival, and LFS were similar in patients receiving PBSCs versus BM.