The transplantation of placental/umbilical cord blood (P/CB) has been used successfully to reconstitute bone marrow function in both related and unrelated recipients. We report here the experience of the New York Blood Center P/CB Program. Since its inception in 1992, over 400 unrelated transplants were supported between July 1993 and September 1997. Overall, event-free survival for all diagnoses and ages approached 0.45. Success and rapidity of engraftment correlated most strongly with the degree of human leukocyte antigen (HLA) disparity and cell dose/kg body weight recipient. Acute graft-versus-host disease (GVHD) was common in all patients but, surprisingly, did not differ between those patients who received grafts having one or more antigen mismatches. Chronic GVHD was uncommon and only rarely contributed to death. These results demonstrate the feasibility of large-scale P/CB banking for the provision of cryopreserved stem cell preparations for unrelated transplants. The degree of the program's success argues strongly for additional P/CB banks in order to increase the likelihood of finding a suitable stem cell preparation for patients for whom related matched donors do not exist.