Expression of inflammatory cytokines derived from Th1 cell population is increased in patients with human T-lymphotropic virus type I (HTLV-I)-associated myelopathy/tropical spastic paraparesis (HAM/TSP). It has been shown that cytokine signaling molecules, including transcription factors T-bet and GATA-3, interleukin-12 receptor beta 2 (IL-12R beta 2) and suppressors of cytokine signaling (SOCS), such as SOCS1, are important in differentiation of naive T cells into Th1 helper T cells. To assess the immunological status from the stand-point of cytokine signaling in patients with HAM/TSP, we analyzed mRNA expression of these cytokine signaling molecules in peripheral blood mononuclear cells using quantitative RT-PCR. Twenty-eight HAM/TSP patients, nine HTLV-I-infected individuals without HAM/TSP and twenty-two HTLV-I-uninfected individuals were included in this study. Expression of T-bet, GATA-3, IL-12R beta 2 and SOCS1 was significantly increased in HAM/TSP patients in comparison with HTLV-I-uninfected individuals. In contrast, expression of SOCS3, a marker for Th2 cells, was significantly decreased in HTLV-I-infected individuals. These results indicate that HAM/TSP patients are associated with increased Th1 and decreased Th2 cytokine signaling activities.