Human T cell lymphotropic virus type I (HTLV-I) provirus load has been reported to be generally higher in patients with HTLV-I-associated myelopathy/tropical spastic paraparesis (HAM/TSP) than in asymptomatic HTLV-I carriers (ACs). However, some ACs have a high HTLV-I provirus load comparable with that in patients with HAM/TSP. To examine whether other factors influence the outcome of HTLV-I infection in patients with HAM/TSP and ACs, we analyzed spontaneous Tax expression and cytokine production in peripheral blood mononuclear cells using flow cytometry. The Tax expression in HTLV-I-infected cells (percentage of Tax expressing cells/HTLV-I provirus load when assumed 1 copy/cell) and the intensity of Tax expression did not differ between these 2 groups. However, the production of interferon-gamma and tumor necrosis factor-alpha in Tax-expressing cells was significantly lower in ACs with high HTLV-I provirus load than in patients with HAM/TSP. This result suggests that the production of inflammatory cytokines in Tax-expressing cells is one of the factors that contribute to the development of HAM/TSP.