ABSTRACT Introduction and hypothesis: Umbilical cord blood (UCB) is an alternative source of hematopoietic stem cells for allogeneic hematopoietic stem cell transplantation in the absence of a compatible donor. The UCB transplantation has a lower incidence of chronic graft versus host disease (GvHD), but is associated with slower engraftment and slower immune reconstitution, compared to other sources. Dendritic cells (DCs) and Natural Killer cells (NKs) play a central role in the development of GvHD and the graft versus leukemia (GvL) effect, as well as in the control of infectious complications. Method: We quantified by multiparametric flow cytometry monocytes, lymphocytes, NK cells, and DCs, including their subsets, in UCB samples from 54 healthy newborns and peripheral blood (PB) from 25 healthy adult volunteers. Results: In the UCB samples, there were higher counts of NK cells 56bright16– (median 0.024 × 109/L), compared to the PB samples (0.012 × 109/L, p < 0.0001), NK 56dim16bright (median 0.446 × 109/L vs. 0.259 × 109/L for PB samples, p = 0.001) and plasmacytoid dendritic cells (pDCs, median 0.008 × 109/L for UCB samples vs. 0.006 × 109/L for PB samples, p = 0.03). Moreover, non-classic monocyte counts were lower in UCB than in PB (median 0.024 × 109/L vs. 0.051 × 109/L, respectively, p < 0.0001). Conclusion: In conclusion, there were higher counts of NK cells and pDCs and lower counts of non-classic monocytes in UCB than in PB from healthy individuals. These findings might explain the lower incidence and severity of chronic GvHD, although maintaining the GvL effect, in UCB transplant recipients, compared to other stem cell sources.