Immunocytochemical staining of spinal cords from five autopsied patients with HTLV-I-associated myelopathy/tropical spastic paraparesis was performed using a panel of monoclonal or polyclonal antibodies reactive with interleukin-1β (IL-1β), interleukin-6 (IL-6), tumor necrosis factor (TNF)-α, interferon (IFN)-α, IFN-β, IFN-γ and transforming growth factor (TGF)-β. In the spinal cords of patients with a shorter duration of illness, IL-lβ, TNF-α, and IFN-γ were expressed on perivascular infiltrating macrophages, astrocytes and microglia in active-chronic inflammatory lesions. In striking contrast, we rarely noted cytokine expression except for IFN-γ in inactive-chronic lesions of patients with longer durations. In situ expression of these cytokines on microglia and astrocytes, in addition to infiltrating mononuclear cells, suggests that glial cells participate in the inflammatory process, especially in active lesions. In addition, the cytokine expression was gradually downregulated along with duration of illness.