This project is part of a Blood and Marrow Transplant Clinical Research Network which is conducting multicenter clinical trials in hematopoietic stem cell transplantation. The purpose of the network is to promote the efficient comparison of novel treatment methods and management strategies of potential benefit for children and adults undergoing blood or marrow transplantation. Studies under development include a comparison of blood versus marrow stem cell transplantation. Studies under development include a comparison of blood versus marrow stem cell transplantation after marrow ablation, the development of non-myeloablative transplant regimens, and the best ways to avoid infections in transplant patients.