In adult patients, regulatory CD4 + FOXP3+ T cells are suggested to have a role in the control of allo-reactive disease after hematopoietic SCT (HSCT). We compared CD4 + FOXP3+ T-cell reconstitution after unrelated cord blood (UCB), matched unrelated donor (MUD) and matched sibling donor (MSD) HSCT in children, starting as early as 1 week after transplantation, and analyzed the association with allo-reactive disease. A total of 30 children were included who underwent a myeloablative-conditioning regimen followed by UCB (12/30), MUD (7/30) or MSD (11/30) HSCT. These three patient groups showed significant differences in FOXP3+ T-cell reconstitution pattern. Early after UCB and MSD, but not after MUD, HSCT a peak in FOXP3+ T cells was observed. There were significant differences in activation status and Ki67 expression of the FOXP3+ T cells after UCB and MSD, respectively. FOXP3+ T-cell proportions early after HSCT and in the graft were inversely correlated with allo-reactivity. This study indicates that FOXP3 reconstitution after HSCT is dependent on the type of graft used. Furthermore, in children evaluation of FOXP3+ T-cell numbers early after HSCT and in the graft may be used to judge the risk of developing allo-reactivity after HSCT.