During human aging, one of the major changes in the T cell repertoire is a dramatic expansion of T cells with the atypical CD28− CD8+ phenotype. In this study, we show that this increase is a consequence not only of an expansion in the CD28− CD8+ population but also of a decrease in the number of CD28+ CD8+ T cells. The decrease in circulating CD28+ CD8+ T cells is dramatically accelerated after the age of 50 and is not accompanied by an equivalent reduction in the CD28+ CD4+ subset. Our findings confirm that aging leads to an accumulation of CD45RO+ T cells within the CD28+ CD8+ subset as previously observed. Surprisingly, we found an increase in CD45RA+ expression with age in the CD28− CD8+ subset. Immune-phenotyping for activation markers, measurement of telomere DNA content, and cytokine production analysis indicate that the large majority of CD28− CD8+ T cells are Ag-experienced, despite their CD45RA+ phenotype. Our study further demonstrates that the poor proliferative response displayed by CD28− CD8+ T cells is not a consequence of telomere shortening. Also, analysis of cytokine production at the single cell level revealed that the proportions of IFN-γ+, IL-4+, and IL-10+ T cells are considerably higher among the CD28− CD8+ than the CD28+ CD8+ subset. In summary, these data explain the presence of CD45RA+ T cells in the elderly, shed light on the phylogenetic origin of CD28− CD8+ T cells, and suggest a role for these cells in the immune senescence process.