It has been shown that human umbilical cord blood contains stem/progenitor cells comparable in number to that of adult bone marrow. We report here the first successful cases of transplantation of umbilical cord blood cells. The patients were suffering from Fanconi's anemia, complicated by severe aplastic anemia. During pregnancy, it was shown that the mother was carrying a sibling unaffected by the disease and with HLA identical to the patient. Cord blood was collected and frozen in liquid nitrogen at birth. After conditioning with low-dose cyclophosphamide (20 mg/kg) and thoraco-abdominal irradiation (5 grays), the patients received a cord blood transplant of thawed cells. Three patients have been transplanted without any immediate side-effect. One has not enough follow-up, but two patients are alive and well with complete donor hematologic reconstitution and no chronic graft versus host disease. Potential developments of this technique are an extension of applicability with regard to other diseases that might be transplanted and whether such transplants can be performed in adults. The relative immaturity of the lymphoid system at birth may be advantageous in decreasing the graft versus host reaction if these cells are used in a mismatched transplantation. Cord blood cell banks may be useful for transplants in patients lacking an HLA-identical donor.