Peripheral blood stem cell transplantation is being used more frequently in both the autologous and allogeneic setting. The use of cytokines either alone or in conjunction with chemotherapy priming has made peripheral procurement of the stem cells through apheresis possible. Differences exist both in the composition of the graft and in subsequent posttransplantation immune reconstitution between peripheral blood stem cells and bone marrow. Reliable estimates of the comparative incidence of acute and chronic graft-versus-host disease following blood or marrow allogeneic transplantation are not yet available.