The first successful transplants of hematopoietic stem cells were done in 1968 in three children with congenital immune deficiency diseases. In each instance, stem cells were collected from the bone marrow of sibling donors who were genotypically HLA identical or closely HLA matched to the recipient. Since then, thousands of patients have received hematopoietic stem cell transplants to treat malignant and non-malignant diseases. Current estimates of annual numbers of stem cell transplants are about 50,000 worldwide, with growth at a rate of 10-15% per year. The reasons for increasing use include proven and potential efficacy in many diseases, better understanding of the appropriate timing of transplantation and patient selection, greater availability of donors and better techniques for determining HLA match, greater ease of stem cell collection, and improved supportive care resulting in less transplant-related morbidity and mortality. About two-thirds of hematopoietic stem cell transplants use autologous cells, generally collected from peripheral blood by leukapheresis. The remainder are allogeneic transplants, most commonly from HLA-identical sibling donors, and most often using cells collected directly from bone marrow.