C D4⁺CD25⁺ regulatory T (T_R) cells have been described in both humans and mice. In mice, T_R are thymically derived, and lack of T_R leads to organ-specific autoimmunity. Recently, the forkhead/winged helix transcription factor, FoxP3, has been shown to be important for the function of T_R cells in mice. In this study, human T_R cells were examined and, in results similar to those of studies done in mice, expression of FoxP3 was found exclusively in CD4⁺CD25⁺ T cells and correlated with the suppressive activity of these cells. In contrast to the mouse studies, activation of human CD4⁺CD25^– T cells led to expression of FoxP3. Expression of FoxP3 in activated human CD4⁺CD25⁺ cells also correlated with suppression of proliferation by these cells in freshly isolated CD4⁺CD25^– T cells from the same donor. This suppression was cell-contact dependent and cytokine independent. Thus, in humans, during activation of CD4⁺CD25^– T cells in an immune response, two populations of cells may arise, effector CD4⁺CD25⁺ and regulatory CD4⁺CD25⁺ T cells, with expression of FoxP3 correlated with regulatory activity. These data also raise the possibility that a failure to generate peripheral T_R cells properly may contribute to autoimmune disease and suggest a possible therapeutic role for FoxP3 in the treatment of such diseases.