Delayed hematopoietic recovery is the main factor precluding a wider use of cord blood (CB) transplants. We hypothesized that this delayed engraftment might not be related to an insufficient number of stem cells in the graft, but to an intrinsic difficulty of these cells to undergo differentiation. To test our hypothesis, 2 groups of children were compared; 12 received a CB transplant and 12 an adult bone marrow (BM) transplant. We studied neutrophil and platelet recovery and, at a median time of approximately 1 year after transplantation, the frequency of colony-forming cells (CFCs) and long-term culture initiating cells (LTC-ICs) in the BM of the 2 groups. Recipients of BM transplants received 1-log more cells and had significantly faster neutrophil and platelet recovery. Conversely, the frequency of committed and early progenitors was significantly higher in the BM of children given CB cells compared with BM transplant recipients (median count of CFC/2 x 10(4) BM mononuclear cells, 20 versus 11, P =.007; median count of LTC-IC/10(6) BM mononuclear cells, 8.2 versus 0.2 P =.001). CB, but not adult BM stem cells, can better restore the host hematopoietic progenitor cell reservoir; the delayed engraftment after CB transplantation may reflect the difficulty of CB progenitors to reprogram themselves toward differentiation.