Large clonal expansions of peripheral CD8+ T cells carrying receptors for single epitopes of CMV and EBV are common in the elderly and may be associated with an immune risk phenotype predicting mortality. Here we show that the frequency of CD8+ T cells expressing the inhibitory killer cell lectin-like receptor G1 (KLRG1), a marker of cells unable to undergo further clonal expansion, was markedly elevated in CD8+ T cells from old donors. Moreover, tetramer staining revealed that the elevated frequency of CMV-specific CD8+ T cells in the elderly was due to an accumulation of cells bearing this dominant negative receptor. The fraction of CMV-specific T cells able to secrete interferon-gamma after specific antigenic stimulation was significantly lower in the elderly than in the young, although the total number of functional cells was comparable. Therefore, the majority of the clonally expanded virus-specific CD8+ cells in the elderly was dysfunctional. Thus, T cell responses are altered in the aged by an accumulation of replicatively senescent dysfunctional T cells carrying receptors for persistent herpes viruses. The presence of clonal expansions of such virus-specific cells may shrink the available repertoire for other antigens and contribute to the increased incidence of infectious disease in the elderly.