In an attempt to reduce the morbidity and mortality associated with allogeneic bone marrow transplantation, clinical investigators in Asia, Australia, Europe, and North America have evaluated umbilical cord and placental blood as an alternate source of hematopoietic stem and progenitor cells for transplantation. To date, umbilical cord blood has been used to reconstitute hematopoiesis in 34 patients with a variety of malignant and nonmalignant diseases treated with myeloablative therapy. Although the median number of nucleated cells infused was only 4.0 x 10(7)/kg of the recipient's body weight (range, 0.1-33.0 x 10(7)), the median time to hematopoietic recovery was 25.0 days for neutrophils (i.e., absolute neutrophil count > 500/microL, range, 12-46) and 43.5 days for platelets (i.e., absolute platelet count > 50,000/microL, range: 15-105+). Except in three patients who failed to demonstrate any evidence of hematopoietic recovery and one patient who had prompt autologous recovery, engraftment of donor cells was documented in all patients, including recipients of HLA-mismatched grafts. Of 23 evaluable patients with HLA-identical or HLA-1 antigen-mismatched donors, none had grade 2-4 acute graft vs. host disease (GVHD). In summary, these data suggest that umbilical cord blood is an acceptable source of transplantable hematopoietic stem cells, at least in recipients < 40 kg and that the risk of acute GVHD is low.