We have addressed whether patients' immune system status after allogeneic stem cell transplantation, assessed more than 1 year after the procedure, recovers normal function as compared with that of their respective donors. An additional aim was to compare the status of the immune system between patients receiving reduced-intensity conditioning regimens and those undergoing myeloablative transplantations. For this purpose, we analyzed not only the different subsets of peripheral blood (PB) lymphocytes, but also circulating dendritic cell (DC) subpopulations, together with cytokine production by PB T cells, in a series of 38 patients undergoing allogeneic stem cell transplantation. We compared these patients with their respective HLA-matched donors by performing a simultaneous patient/donor paired study. Complete bone marrow chimerism status and normal PB cell counts were demonstrated in all recipients. The most relevant numeric differences found between patients and donors were related to the distribution of the distinct subsets of PB DCs (CD16+ DCs were increased, whereas myeloid and plasmacytoid DC subsets were decreased in the patient group). This was associated with an increased number of B cells, an inverted CD4/CD8 T-cell ratio, and a decrease in CD4+/CD8+ double-positive T cells in the patient group. In addition, a predominance of a T-helper 1 pattern of cytokine production (interferon gamma and tumor necrosis factor alpha) with decreased secretion of T-helper 2-associated cytokines (interleukin 5 and interleukin 10) was also observed at the single-cell level. No significant differences were found in any of the parameters analyzed between patients receiving reduced-intensity conditioning regimens and those undergoing myeloablative transplantations.