Excessive T helper type 1 (Th1) cell activity has been reported in Behçet's disease (BD). Recently, association of Th17 cells with certain autoimmune diseases was reported, and we thus investigated circulating Th17 cells in BD. CD4+CD45RO– (naive) T cells were cultured with Th0-, Th1-, Th2- and Th17-related cytokines and antibodies, and their mRNA was studied by real-time polymerase chain reaction (PCR). When naive CD4+ T cells were cultured with Th1- and Th17-related cytokines, interferon (IFN)-γ mRNA and interleukin (IL)-17 mRNA were up-regulated, respectively, in BD patients. Naive CD4+ T cells cultured in a Th17 cell-inducing condition expressed IL-23 receptor (IL-23R) mRNA excessively. IL-17 mRNA expression was induced only when naive CD4+T cells were cultured in the presence of IL-23. CD4+ T cells cultured with Th17 cytokines expressed excessive RAR-related orphan receptor C (RORC) mRNA. Using intracellular cytokine staining, we found that CD45RO+(memory) CD4+ T cells producing IL-17 and IFN-γ simultaneously were increased significantly. Memory CD4+ T cells producing IFN-γ but not IL-17 decreased profoundly in BD patients. CD4+ T cells producing IL-17 and IFN-γ simultaneously were found in BD skin lesions. Collectively, we found excessive CD4+ T cells producing IL-17 and IFN-γ (Th1/Th17) cells in patients with BD, and possible involvement of IL-23/IL-23R pathway for the appearance of excessive Th1/Th17 cells.