One of the consequences of HIV infection is damage to the CNS. To characterize the virologic, immunologic, and functional factors involved in HIV-induced CNS disease, we analyzed the viral loads and T cell infiltrates in the brains of SIV-infected rhesus monkeys whose CNS function (sensory evoked potential) was impaired. Following infection, CNS evoked potentials were abnormal, indicating early CNS disease. Upon autopsy at 11 wk post-SIV inoculation, the brains of infected animals contained over 5-fold more CD8+ T cells than did uninfected controls. In both infected and uninfected groups, these CD8+ T cells presented distinct levels of activation markers (CD11a and CD95) at different sites: brain> CSF> spleen= blood> lymph nodes. The CD8+ cells obtained from the brains of infected monkeys expressed mRNA for cytolytic and proinflammatory molecules, such as granzymes A and B, perforin, and IFN-γ. Therefore, the neurological dysfunctions correlated with increased numbers of CD8+ T cells of an activated phenotype in the brain, suggesting that virus-host interactions contributed to the related CNS functional defects.