The number of unrelated cord blood transplants is increasing, with more than 8000 patients reported worldwide. Criteria of donor choice have been identified. Cell dose measured by number of nucleated cells is the most important factor, thus increases in cell dose can partially overcome the presence of HLA incompatibilities. Overall, increasing the number of HLA incompatibilities is associated with non-engraftment, but it also decreases the risk of relapse in patients with malignant disease, resulting in an absence of effect of HLA incompatibilities on event-free survival (survival without relapse or complications related to the transplant). However, in patients with non-malignant disorders, increasing the number of HLA incompatibilities decreases the overall survival. These results show that criteria of donor choice based on number of infused cells, number of HLA incompatibilities and diagnosis improve outcomes of unrelated cord blood transplants. Currently, owing to the possibility of using a non HLA identical donor, most patients can find a donor, increasing the need for the development of the inventory of cord blood banks. Methods of improving engraftment are currently under investigation.