Cancer of the colon is one of the leading causes of cancer death in Western countries and is increasing rapidly in Japan. Epidemiological and laboratory animal model studies have suggested an inverse relationship between colon cancer risk and intake of fiber-rich foods. The protective effect of dietary fiber which comprises a heterogeneous group of nonstarch polysaccharides such as cellulose, hemicellulose, and pectin and noncarbohydrate substances such as phytic acid depends on the nature and source of fiber in the diet. Laboratory animal models have consistently shown that dietary administration of wheat bran reduced colon tumorigenesis. Human diet intervention studies have demonstrated that supplemental wheat bran in the diet decreased the formation of putative metabolites such as secondary bile acids and diacylglycerol in the colon that have been shown to act as tumor promoters in the colon. Among the components of dietary fiber, especially wheat bran, phytic acid (inositol hexaphosphate) has been studied extensively for its chemopreventive properties against colon carcinogenesis in the laboratory animal models. In studies carried out to date, dietary phytic acid reduced the incidence of colonic aberrant crypt foci, putative preneoplastic lesions in rats. Oral administration of phytic acid was shown to inhibit colon carcinogenesis in rodents during the initiation and postinitiation stages. These studies provide evidence for potential chemopreventive properties of phytic acid against colon cancer. With regard to mode of action, phytic acid acts as an antioxidant, to reduce the rate of cell proliferation and to augment the immune response by enhancing the activity of natural killer (NK) cells.