Natural simian immunodeficiency virus (SIV) infection in sooty mangabeys (SMs) typically does not result in AIDS, despite high-level viremia and significant depletion of mucosal CD4⁺ T cells. Here, we report the results of the first longitudinal study of a large cohort of SMs naturally infected with SIV (n = 78) housed at the Yerkes National Primate Research Center from which samples were obtained three times over a 5-year period. In this study, we observed (i) no signs of simian AIDS, (ii) stable SIV loads, (iii) a slow but progressive decline in CD4⁺ T-cell counts (from a mean of 1,067.0 cells/mm³ at time point 1 to 764.8 cells/mm³ at time point 3) and increases in the numbers of animals with CD4⁺ T-cell levels below 500 and 200 cells/mm³ (from 8 to 28 of 78 and from 1 to 4 of 78, respectively), (iv) progressive declines in percentages of naïve CD4⁺ and CD8⁺ T cells (from 37.7 to 24.8% and from 21.0 to 13.0%, respectively), and (v) stably low levels of activated/proliferating T cells as well as CD4⁺ CCR5⁺ T cells. Since the level of total CD4⁺ T cells and the fraction of naïve T cells in SIV-uninfected SMs also declined, it is possible that some of these observations are related to aging, as the SIV-infected animals were significantly older than the uninfected animals. In contrast to the decline in CD4⁺ T cell counts in individuals infected with human immunodeficiency virus (HIV), the decline in CD4⁺ T cell counts in SMs naturally infected with SIV over a 5-year period was not predicted by either plasma viremia or levels of T-cell activation. Taken together, these results confirm that natural SIV infection is nonprogressive from a clinical, virological, and immunological point of view and that stable levels of viremia associated with persistently low-level immune activation represent key differences from the natural course of HIV infection in humans.