To address the possible role of replicative senescence in human immunodeficiency virus (HIV) infection, telomere length, telomerase activity, and in vitro replicative capacity were assessed in peripheral blood T cells from HIV+ and HIV- donors. Genetic and age-specific effects on these parameters were controlled by studying HIV-discordant pairs of monozygotic twins. Telomere terminal restriction fragment (TRF) lengths from CD4+ T cells of HIV+ donors were significantly greater than those from HIV- twins. In contrast, telomere lengths in CD8+ T cells from HIV+ donors were shorter than in HIV- donors. The in vitro replicative capacity of CD4+ cells from HIV+ donors was equivalent to that of HIV- donors in response to stimulation through T cell receptor CD3 and CD28. Little or no telomerase activity was detected in freshly isolated CD4+ or CD8+ lymphocytes from HIV+ or HIV- donors, but was induced by in vitro stimulation of both HIV+ and HIV- donor cells. These results suggest that HIV infection is associated with alterations in the population dynamics of both CD4+ and CD8+ T cells, but fail to provide evidence for clonal exhaustion or replicative senescence as a mechanism underlying the decline in CD4+ T cells of HIV-infected donors.