Use of bone marrow transplant therapy has been limited both by the availability of suitable stem cell donors as well as the various immunologic complications, namely graft-versus-host disease (GVHD) and profound immunosuppression resulting in opportunistic infection. As of March 1997, more than 450 patients with various malignant and nonmalignant diseases have been transplanted with umbilical cord blood from related and unrelated donors. Preliminary results suggest that rate of engraftment is high and risk of severe GVHD is low, regardless of donor type. For recipients of HLA-matched or HLA-1 antigen-mismatched sibling donor umbilical cord blood grafts (n = 62) as reported to the International Cord Blood Transplant Registry, the probabilities of engraftment and grade III-IV GVHD are 0.91 +/- 0.02 and 0.01 +/- 0.01, respectively. For recipients of HLA-matched or HLA 1-3 antigen-mismatched unrelated donor umbilical cord blood grafts (n = 85) transplanted at Duke University and University of Minnesota, the probabilities of engraftment and grade III-IV GVHD are 0.94 +/- 0.08 and 0.10 +/- 0.03, respectively. Notably, extensive chronic GVHD has yet to be reported in either series. The overall survival in recipients of unrelated donor umbilical cord blood grafts is 0.40 +/- 0.12 at 2 years after transplantation. Notably, no risk factor predicted higher risk of GVHD and only cell dose (P = 0.04) and possibly, degree of HLA mismatch (P = 0.06) impacted survival in multiple regression analysis. Although these data verify earlier reports that sufficient numbers of hematopoietic stem cells exist in umbilical cord blood for most recipients and that risks of acute and chronic GVHD are low despite increased HLA disparity, these data also provide us with a statistical analysis indicating which demographic, graft, and treatment factors possibly influence various outcomes after transplantation.