The stem cell banking system in Japan by the Japan Marrow Donor Program (JMDP) and Japan Cord Blood Bank Network (JCBBN) has provided increased opportunities for patients who might benefit from stem cell transplant from allogeneic sources but who lack human leukocyte antigen (HLA)-matched related donors. Nevertheless, most patients probably do not undergo transplantation because of the absence of suitable stem cell sources. To fulfill this potential need, the outcomes of transplants from HLA-mismatched relatives with or without T-cell depletion were retrospectively analyzed: the rates of engraftment and survival were insufficient in transplants with T-cell depletion, and the actual increase in transplantable donor numbers was small because only a single locus mismatched donor was the realistic choice in those without T-cell depletion. Since prophylaxis with tacrolimus reduced the incidence of grade I-IV acute graft-versus-host disease (GVHD) in HLA class I allele mismatched unrelated donors, we studied transplantation from HLA one haploidentical family donors who showed microchimerism of noninherited maternal antigens, without T-cell depletion but with tacrolimus prophylaxis. The rates of engraftment and survival in this circumstance were similar to those obtained with transplantation from HLA-matched sibling donors.