Elevated CD4 T-cell turnover may lead to the exhaustion of the immune system during human immunodeficiency virus type 1 (HIV-1) and simian immunodeficiency virus (SIV) infections. However, this hypothesis remains controversial. Most studies of this subject have concerned the blood, and information about the lymph nodes is rare and controversial. We used Ki67 expression to measure cycling T cells in the blood and lymph nodes of uninfected macaques and of macaques infected with a pathogenic SIVmac251 strain or with a nonpathogenic SIVmac251Δnef clone. During the asymptomatic phase of infection, the number of cycling CD8⁺ T cells progressively increased (two- to eightfold) both in the blood and in the lymph nodes of macaques infected with SIVmac251. This increase was correlated with viral replication and the progression to AIDS. In contrast, no increases in the numbers of cycling CD4⁺ T cells were found in the blood or lymph nodes of macaques infected with the pathogenic SIVmac251 strain in comparison with SIVmac251Δnef-infected or healthy macaques during this chronic phase. However, the lymph nodes of pre-AIDS stage SIVmac251-infected macaques contained more cycling CD4⁺ T cells (low baseline CD4⁺-T-cell counts in the blood). Taken together, these results show that the profiles of CD4⁺- and CD8⁺-T-cell dynamics are distinct both in the lymph nodes and blood and suggest that higher CD4⁺-T-cell proliferation at the onset of AIDS may lead to the exhaustion of the immune system.