Progress in comprehension of the immunogenetics of the HLA-complex and the discovery of new very potent immunosuppressive agents have enabled organ and tissue transplantations to be performed as a relatively routine therapeutic method. Long-term outcome measured as a half-life in kidney transplantations is 25 years among HLA-identical siblings, 12 years in one haplotype-mismatched paternal donors, and 7 years in cadaver transplantation program. The long-term outcome in the latter group can be markedly improved--to as much as 19 years--when six-antigen program is observed (i.e. donor and recipient are identical in HLA-DR, -B, and -A antigens). The survival of patients after bone marrow transplantation (BMT) having as a donor a HLA-identical sibling has improved remarkably, too. However, only about 30 percent of patients who might benefit from a bone marrow transplant have a genotypically. HLA-identical sibling who could be a donor. Transplants from unrelated donors have become therefore an alternative method. To find HLA-matched donors the establishment of large registries are needed. One of them--Them Bone Marrow Donors Worldwide--in Leiden has over 1.2 million of potential donors at the time being (1992).