BACKGROUND: Interleukin 25 (IL-25) is involved in the initiation of T helper cell (Th)2-mediated immunopathologies. In this study, we investigated the expression of IL-25 in inflammatory bowel disease (IBD) and its role in the induction of CD4 T-cell differentiation. METHODS: Expression of IL-25 in inflamed mucosa of patients with IBD was determined by quantitative real-time polymerase chain reaction, immunohistochemistry, and enzyme-linked immunosorbent assay. The correlation of IL-25 expression with endoscopic disease activities and C-reactive protein was evaluated. Peripheral blood and lamina propria CD4 T cells were stimulated with anti-CD3 and anti-CD28 monoclonal antibodies in the presence of IL-25. Transcription factors and cytokines were determined with real-time polymerase chain reaction, flow cytometry, and enzyme-linked immunosorbent assay. RESULTS: IL-25 was significantly decreased in the sera and inflamed mucosa of patients with active IBD compared with controls. It was upregulated in the sera of patients with Crohn's disease after treatment with infliximab. The levels of IL-25 in inflamed mucosa and sera were inversely correlated with endoscopic disease activities and C-reactive protein, respectively, in IBD. IL-25 could markedly inhibit IBD CD4 T cells to produce tumor necrosis factor, interferon γ, and IL-17A but promote IL-10 secretion. It suppressed the differentiation of IBD CD4 T cells into Th1 and Th17 cells but did not interfere with Th2 cell differentiation. Importantly, blockade of IL-10 secretion by IBD CD4 T cells markedly attenuated the inhibitory role of IL-25 in modulating both Th1 and Th17 immune responses. CONCLUSIONS: IL-25 is markedly decreased in IBD and inhibits IBD CD4 T-cell activation and differentiation into Th1/Th17 cells in an IL-10-dependent manner, suggesting that it may be a potential therapeutic agent for IBD.