Our previous study indicated that the interleukin (IL)-6/STAT-3 signal was up-regulated in inflammatory bowel disease (IBD) in both humans and animal models. We also discovered phosphorylated STAT-3 in the nucleus of the colonic epithelial cells in IBD mice. Intestinal epithelial cells (IEC) have been shown to secrete IL-6. Therefore, the secretion of IL-6 from IEC may be one of the mechanisms of STAT-3 phosphorylation in IEC during the pathogenesis of IBD, and inhibition of IL-6 production by IEC may be beneficial in preventing IBD. We examined the preventative effect of various types of fucoidans on IL-6 production in a lipopolysaccahride (LPS)-stimulated murine colonic epithelial cells line, CMT-93, in vitro. We also determined in vivo the effect of fucoidans on murine chronic colitis induced with dextran sodium sulphate. Among fucoidans, those from Cladosiphon okamuranus Tokida and Kjellmaniella crassifolia inhibited IL-6 production in CMT-93 cells with the down-regulation of NF-κB nuclear translocation. Analysis of the effect of fucoidan on murine colitis in vivo showed that the disease activity index and myeloperoxidase activity decreased in mice fed Cladosiphon fucoidan, but not Fucus fucoidan. Cytokine profiles in colonic lamina propria indicated that the synthesis of interferon (IFN)-γ and IL-6 decreased and that of IL-10 and transforming growth factor (TGF)-β increased in mice fed Cladosiphon fucoidan, compared with mice fed a standard diet or Fucus fucoidan. The levels of IL-6 mRNA in colonic epithelial cells was lower in colitis-induced Balb/c mice fed Cladosiphon fucoidan than those fed a standard diet. Fucoidan improves murine chronic colitis by down-regulating the synthesis of IL-6 in the colonic epithelial cells. Fucoidan derived from C. o. Tokida may be useful as a dietary substance for the patients with inflammatory bowel disease.