Because of the anergy of CD25+CD4+ regulatory T cells, it is unclear how the number of these regulatory T cells is sustained and expanded in normal physiologic circumstances. In the present study, we examined the effect of natural allogeneic mature dendritic cells (DCs) on the proliferation and function of CD25+CD4+ T cells. Our data showed that natural allogeneic mature DCs stimulated CD25+CD4+ T-cell growth vigorously, whereas immature DCs had little effect on the proliferation of CD25+CD4+ T cells. After expansion by mature DCs, CD25+CD4+ T cells maintained their expression of Foxp3 and suppressed the proliferation of CD25- CD4+ T cells similar to freshly isolated CD25+CD4+ T cells. Our results introduce a potentially critical role played by natural allogeneic mature DCs, which exist in normal physiologic circumstances, in controlling CD25+CD4+ regulatory T-cell expansion and function.