Monoclonal anti-CD25-antibodies are successfully applied in organ transplantation to reduce the incidence of acute graft rejection. However, targeting the CD25 molecule might not only affect activated T-cells but also regulatory T-cells (T(regs)) constitutively expressing the CD4(+)CD25(+)CD127(low)FoxP3(+) phenotype. In this study, we investigated the influence of the anti-CD25-antibody Basiliximab on the frequency of T(regs) early after kidney transplantation comparing individuals receiving/not receiving induction therapy (n = 14 and n = 7). Following Basiliximab administration, a distinct loss of CD4(+)CD25(high) T-cells was observed lasting for at least 6 weeks. This was not accompanied by a disappearance of the entire CD4(+)CD25(+)FoxP3(+) T(regs) but rather a decreased expression density of CD25 on the latter. In addition, a transient rise in CD4(+)CD25(-)FoxP3(+) T-cells was found which expressed the CD127(low) phenotype. Thus, a phenotypic shift of T(regs) from the CD25(+) to the CD25(-) compartment was suggested. This was supported by in vitro findings showing that the disappearance of CD4(+)CD25(high) cells in the presence of Basiliximab was due to down-regulation of CD25 expression meanwhile the suppressive function of these cells was maintained. In conclusion, Basiliximab therapy directly affects CD4(+)CD25(+)CD127(low)FoxP3(+) T(regs) but does not seem to be associated with functional consequences. Thus, it is unlikely that Basiliximab treatment negatively influences strategies involving T(regs) to promote tolerance after organ transplantation.