Mast cells play a key role in the pathophysiology of inflammatory bowel disease (IBD). Tranilast, a mast cell stabilizer, has been empirically used for IBD in Japan, but its precise role in the treatment of IBD is largely unknown. To investigate the role of tranilast for the treatment of IBD, tranilast was administered intrarectally to mice with dextran sulfate sodium (DSS)-induced colitis. Tranilast ameliorated DSS colitis clinically and pathologically, as demonstrated by decreased number and degranulation of mast cells in the colon. mRNA expression was increased for tumor necrosis factor-α, interferon-γ and interleukin (IL)-6, and decreased for IL-10 in the colon of DSS colitis mice. In contrast, tranilast markedly decreased expression of mRNAs for the pro-inflammatory cytokines, and increased that of the anti-inflammatory cytokines. Moreover, tranilast increased heme oxygenase (HO)-1 expression on colonic epithelial cells as well as on colon-infiltrating cells of DSS colitis. In conclusion, tranilast ameliorated DSS colitis by regulating mast cell degranulation, decreasing inflammatory cytokines and increasing anti-inflammatory cytokines. Tranilast might exert these effects partly through enhanced HO-1 expression in the colon, suggesting a potential adjunctive therapy for IBD. © 2009 Japanese Society of Pathology.