Aging is associated with a progressive decline in T cell-mediated immune responses. However, it has been unknown whether regulatory/suppressive CD4 T cells are involved in this decline. Our in vitro analyses revealed that CD4+ CD25+ T cells, the well-characterized naturally occurring regulatory/suppressive CD4 T cells, in aged mice are functionally comparable to those in young mice (ie, anergic and suppressive), although slightly increased in number. In contrast, functional changes to whole CD4+ CD25− T cells were pronounced in aged mice, ie, the majority of aged CD4+ CD25− T cells exhibited a significant hyporesponsiveness, and the remaining cells maintained a normal responsiveness. Furthermore, we identified Foxp3 (a transcription factor critical in conferring the regulatory/suppressive function to CD4 T cells)-positive suppressive CD4 T cells among aged hyporesponsive CD4+ CD25− T cells. These results suggest that the age-related decline in T cell-mediated immune responses is ascribable to changes in the CD4+ CD25− T cell population and not to a functional augmentation of suppressive CD4+ CD25+ T cells.