Human umbilical cord blood is an alternative source of hematopoietic stem and progenitor cells capable of reconstituting the bone marrow of recipients with a variety of diseases. Cord blood stem and progenitor cells from related donors have been successfully transplanted world-wide in children with genetic or hematologic diseases. These results suggested that cord blood from unrelated, HLA-matched donors also can be used for patients who need a transplant but don't have a related donor. The existing problems in unrelated-donor bone marrow transplantation with donor recruitment, bone marrow harvesting, and matching for antigens peculiar to a particular ethnic group would be reduced if typed, tested, and frozen umbilical cord blood could be made easily and rapidly available. Such a system, which could complement or partially replace the present process, has been shown to be feasible. In an NHLBI-sponsored program, investigators have collected and cryopreserved a bank of about 9,000 human UCBUs. In addition, successful searches and matches (including many 4/6 antigen matches) have resulted in more than 900 unrelated-donor, UCBU transplants to date. Preliminary data from both these unrelated-donor and other related-donor cord blood transplants suggest that there is less GvHD than if the source of the graft were adult bone marrow. The severity of GvHD also seems less even in highly mismatched recipients. One important question is whether or not 3/6 HLA matched transplants in children will have acceptable 180-day disease free survival. It also remains to be determined if the graft-vs-leukemia effect is also less or if unrelated-donor cord blood grafts are as durable as those from matched, related-donors. Most transplants conducted thus far have been in children. Hence another important question is whether or not there are sufficient numbers of stem cells in cord blood to support transplantation to an unrelated adult.