It is thought that human T-cell lymphotropic virus type I (HTLV-I) preferentially infects CD4⁺ T cells in vivo. However, observations of high HTLV-I proviral load in patients with HTLV-I–associated myelopathy/tropical spastic paraparesis suggest that HTLV-I may infect other cell types in addition to CD4⁺ T cells. To identify in vivo T-cell tropisms of HTLV-I, real-time quantitative polymerase chain reaction (PCR) and intracellular protein staining were used. A high amount of HTLV-I proviral DNA was detected from purified CD8⁺ T cells by quantitative PCR (between 1.64 and 62.83 copies of HTLV-I provirus per 100 isolated CD8⁺ T cells). CD8⁺ T cells expressed HTLV-I–related antigens (HTLV-I Tax and p19 protein) after a short time in cultivation. These results demonstrate that CD8⁺ T cells are also infected with HTLV-I and express HTLV-I antigens at levels that are comparable to HTLV-I–infected CD4⁺ cells. Therefore, CD8⁺ cells are an additional viral reservoir in vivo for HTLV-I and may contribute to the pathogenesis of HTLV-I–mediated disorders.