Crohn's disease and ulcerative colitis, also called chronic inflammatory bowel diseases (IBD), affect up to 500 per 100,000 persons in the Western world. Recent studies in the etiology of IBD suggest that these diseases are caused by a combination of genetic, environmental, and immunological factors. Results from humans and especially animal models of colitis reported by our group and others have indicated that these diseases result from a lack of tolerance to resident intestinal bacteria in genetically susceptible hosts. Probiotic bacteria have health-promoting effects for the host when ingested and have also shown efficacy in ulcerative colitis and refractory pouchitis. In light of the efficacy of providing probiotic bacteria to patients with IBD, there has been interest in the prophylactic and therapeutic potential of inulin, oligofructose, and other prebiotics for patients with or at risk of IBD. Prebiotics are nondigestible dietary oligosaccharides that affect the host by selectively stimulating growth, activity, or both of selective intestinal (probiotic) bacteria. Prebiotics are easy to administer and, in contrast to probiotic therapy, do not require administration of large amounts of (live) bacteria and are therefore easier to administer. Studies using prebiotics, especially beta-fructan oligosaccharides, for the treatment of chronic intestinal inflammation have shown benefit in animal models of colitis. Studies using these prebiotics alone or in combination with probiotics are emerging and have shown promise. These dietary therapies could lead to novel treatments for these chronic debilitating diseases.