As the number of successful marrow transplants has increased, the lack of HLA-identical sibling donors for 60 to 70 percent of transplant candidates has become a serious problem. Pilot studies established that marrow transplantation between phenotypically HLA-identical, unrelated individuals can be accomplished successfully. Therefore, the National Marrow Donor Program was established to develop a large file of volunteer marrow donors and to serve as a center for the coordination of the donor search and donor-recipient matching processes. By November 1991, 63 months after the program was established, 457,205 potential marrow donors typed for HLA-A and -B antigens had agreed to be listed in the marrow donor registry. A donor search had been initiated for 8481 patients. At least one potential donor matched for at least three of the four HLA-A and -B antigens was located for 99.8 percent of patients. Among the 3156 searches that were completed, 940 (29.8%) resulted in a transplant. The median time in which to locate a matched donor, complete all predonation evaluations, and obtain donor consent was 208 days. The most common diagnosis in patients who underwent transplantation was chronic myelogenous leukemia (42.0%). When this analysis was completed in November 1991, the National Marrow Donor Program was operating a national network of 99 donor centers and 53 transplant centers. The donor file was increasing rapidly, and a follow- up system was in place to determine the effects of donation on the donors and the outcome in the patients who underwent transplantation. This national network of donor and transplant centers exists and is now facilitating unrelated-donor marrow transplants. The National Marrow Donor Program made it possible to locate donors for many patients in need of a transplant and helped to determine the role of unrelated- donor marrow transplants in the treatment of many diseases.