We have analyzed the kinetics of reconstitution of circulating dendritic cell (DC) subsets (myeloid-DC1 and lymphoid-DC2) in 19 patients affected by acute leukemia undergoing allogeneic hematopoietic stem-cell transplantation (HSCT). We have found that pretransplant DC1 and DC2 were lower in leukemic patients than in healthy subjects (P = 0.003 and P = 0.004, respectively) and that the number of DC2 (but not DC1) infused with the graft was higher in patients receiving peripheral blood stem cells (PBSC) (P = 0.03). Patients recovered to the pretransplant DC1 and DC2 levels within day +60; however, a normal DC1 number was reached on day +365, while DC2 remained lower than in controls up to 1 yr after transplant. DC1 reconstitution did not differ significantly between patients receiving bone marrow stem cells (BMSC) or PBSC, while patients receiving PBSC presented increased levels of DC2 on day +30 (P = 0.008) and +100 (P = 0.047) and a higher number of T lymphocytes and natural killer cells until day +365. The occurrence of graft vs. host disease (GVHD) was not influenced in our cases by DC1/DC2 graft composition, but patients with acute GVHD when compared with patients without acute GVHD presented a significantly less rapid DC recovery (DC1 P = 0.03, DC2 P = 0.009 on day +30, and DC1 P = 0.012, DC2 P = 0.006 on day +100). At the moment of relapse, a decrease of DC1/DC2 numbers was observed in four patients and the presence of two different DC populations one with a normal karyotype, and the other with the same cytogenetic abnormality as the malignant clone was detected by fluorescence in situ hybridization analysis. In conclusion, these observations suggest that in allogeneic HSCT recipients, DC recovery is a slow process possibly contributing to the high risk of infections in the post-transplant period and is possibly influenced by the source of HSC, the occurrence of GVHD and relapse. Further studies are warranted to investigate the significance of DC reconstitution in the transplant setting.