The major obstacle to the therapeutic use of hematopoietic transplantation is the unavailability of matched, unrelated marrow donors for the large number of potential patients, although all of them have the chance to find sufficiently matched, unrelated cord blood units. However, the use of cord blood as a source of cells for transplantation is limited by its cell number, usually below 1 billion, which allows for routine transplantation only in children weighting less than 30 kg, while most potential recipients possess a higher body mass. This led to the idea of the simultaneous use of several units of cord blood which, combined, would fulfill the requirements for the necessary cell number for an adult recipient. We attempted to simultaneously transplant an adult patient with refractory acute myeloblastic leukemia utilizing two different cord blood units, one fully matched and one mismatched at one locus. The patient became reconstituted with only one unit, the mismatched, as determined using microsatellite markers, and had no signs of relapse of leukemia. Unfortunately, he died of persistent fungal (brain aspergilloma) infection on day +103. The successful engraftment may suggest that a method based on the principle of using more than one cord blood unit for transplantation is feasible in large adult patients and may reach routine application.