The number of infused cells is a very important factor in cord blood transplant (CBT) engraftment. Prior ex vivo expansion of aliquots of transplanted cord blood (CB) units is being investigated as a procedure to increase engraftment potential, but results are difficult to evaluate due to a lack of markers for assessing the contribution of expanded cells. We transplanted five patients, infusing the best available CB unit and cells from a second donor simultaneously. In two patients, these cells were obtained from another frozen CB unit by CD34+ positive selection and culture expansion; the other three patients received uncultured highly purified haploidentical CD34+ cells. The first two patients had DNA from the culture expanded CB cells detected only for a few days around day+ 11 when the absolute neutrophil count (ANC) was> 200/μl; thereafter and when the ANC was< 500/μl, only donor DNA from the uncultured CB was detected. For the other three patients, DNA analysis showed early and transient granulocyte engraftment of haploidentical cells, progressively replaced by the CB-derived granulocytes. We concluded that:(1) simultaneous infusion of lymphocyte-depleted HLA highly mismatched haematopoietic progenitor cells has not produced unfavourable effects for CBT;(2) the double transplant model is suitable for evaluating the engraftment potential of ex vivocultured CB cells in the clinical setting;(3) the culture conditions used did not result in early recovery of ANC; and (4) co-transplantation of purified uncultured HLA haploidentical CD34+ cells may reduce the time of neutropenia following CBT. Bone Marrow Transplantation (2001) 28, 355–363.