Human leukocyte antigen-disparity is an essential factor in selecting a suitable donor for allogeneic bone marrow transplantation (BMT), and selection criteria may differ between countries or races and between adults and children. We investigated the usefulness of partially mismatched related donors (PMRD) for Japanese children in comparison with matched sibling donors (MSD) and matched unrelated donors (MUD). Eighteen patients were transplanted from PMRD, who consisted of 12 parents, five siblings, and one cousin. Five of these 18 patient-donor pairs were serologically two-loci mismatched and 13 were one-locus mismatched. The probability of engraftment from PMRD was not different from that using BMT from MSD (n = 59) or MUD (n = 28). Severe acute graft-versus-host disease (GVHD) (>/=grade III) developed more frequently in PMRD (25.5 +/- 11.0%) than in MSD (0.0%), but was seen just as often as in MUD (21.9 +/- 7.9%). The probabilities of chronic GVHD in PMRD (56.7 +/- 14.3%) and MUD (41.7 +/- 11.4%) were significantly higher than that in MSD (18.7 +/- 5.7%, p = 0.01). However, there was no difference in the probability of event-free survival among the three groups. We conclude that PMRD (up to two-loci mismatch) could become suitable donors in BMT to the same extent as MUD for pediatric patients in Japan.