It has recently been shown that CD4(+) CD25(+) T cells are immunoregulatory T cells that prevent CD4(+) T cell-mediated organ-specific autoimmune diseases. To determine whether CD4(+) CD25(+) T cells downregulate Th2 cell-mediated allergic inflammation in the airways, we studied antigen-induced eosinophil recruitment in the airways in BALB/c Rag-2(-)(/-) mice transferred with CD4(+) CD25(+) T cell-depleted or unfractionated T cells from ovalbumin-specific TCR transgenic mice. Antigen-induced eosinophil recruitment into the airways was significantly decreased in the mice transferred with CD4(+) CD25(+) T cell-depleted splenocytes as compared with those transferred with unfractionated splenocytes. On the other hand, the depletion of CD4(+) CD25(+) T cells increased antigen-induced neutrophil and T cell recruitment in the airways of the mice. The depletion of CD4(+) CD25(+) T cells also decreased antigen-induced IL-4 and IL-5 production in the airways of the mice. Finally, the depletion of CD4(+) CD25(+) T cells prevented antigen-induced Th2 cell differentiation in vitro but increased the differentiation of Th1 cells. These results indicate that CD4(+) CD25(+) T cells modulate the Th1 and Th2 cell balance toward Th2 cells and thus upregulate Th2 cell-mediated allergic inflammation in the airways.