Human T lymphotropic virus type I (HTLV-I)-associated myelopathy/tropical spastic paraparesis (HAM/TSP) is an inflammatory neurologic disease caused by HTLV-I infection and has been associated with elevated levels of several proinflammatory cytokines in both serum and cerebrospinal fluid. It is unknown what kind of cells secrete these cytokines and if HTLV-I Ags are associated with this phenomenon. Here, we investigated the expression of cytokines in PBL from eight HAM/TSP patients, nine HTLV-I-infected asymptomatic carriers, and seven healthy controls by flow cytometry combined with intracellular cytokine staining. PBL were cultured with brefeldin A without mitogen and IL-2 for 14 h. Under these conditions, CD8+ cells produced proinflammatory cytokines including IFN-γ, TNF-α, and IL-2, which were significantly elevated in HAM/TSP patients. The proportion of CD8+ cells producing IFN-γ in HAM/TSP patients, asymptomatic carriers, and healthy controls were, on average, 4.9, 0.4, and 0.3%, respectively. IFN-γ production by these CD8+ cells was suppressed by anti-HLA-class I Ab. Purified CD8+ cells from an HLA-A2 HAM/TSP patient produced IFN-γ by cocultivation with autologous CD4 cells, the main reservoir of HTLV-I in vivo, or allogenic HLA-A2+ B cells pulsed with a known immunodominant HTLV-I tax peptide. These data suggest that high levels of circulating HTLV-I-specific CD8+ T lymphocytes have the potential to produce proinflammatory cytokines and may promote inflammatory responses to HTLV-I in HAM/TSP patients.