It has been reported that low-dose of tacrolimus (Tac) is advantageous for the long-term allograft function and survival when compared with standard-dose of tacrolimus. However, the underlying mechanism is not known and remains to be elucidated. CD4(+)CD25(+)FoxP3(+) regulatory T cells (Tregs) play a key role in the induction and maintenance of transplantation tolerance. We studied whether low-dose of Tac would favor the induction of donor-specific Tregs. We found that in all transplant recipients treated with low-dose of Tac (target trough level of 3 to 7 ng/ml), CD4(+)CD25(+)FoxP3(+) Tregs were induced and expanded in the periphery and accumulated in the allograft after transplantation, and they retained their suppressive efficacy in vitro. When studied in vitro, we found that high concentration of Tac significantly decreased the induction of FoxP3 expression in mixed lymphocyte reactions (MLR) when compared with low concentration of Tac. Taken together, these data imply that in solid-organ transplantation the minimization of Tac might be beneficial and favors the induction of donor-specific Tregs maintaining transplantation tolerance to alloantigen.