Peripheral blood stem cells have largely replaced bone marrow as the source of cells in autologous transplantation because of more rapid neutrophil and platelet recovery and faster immune reconstitution. Allogeneic peripheral blood stem cells similarly lead to faster hematologic recovery: however, their effects on graft-vs.-host disease, relapse, survival, and immune reconstitution have been less certain. Eight randomized trials have been published to date comparing the clinical outcomes of allogeneic-related donor bone marrow transplantation (BMT) vs. PBSCT and will be reviewed. In addition, comparisons between the two stem cell sources in unrelated donor transplantation and the increasingly utilized nonmyeloablative transplantation will be discussed.