Background and Aims: Starch that escapes digestion in the small intestine (resistant starch [RS]) may act similarly to nonstarch polysaccharides in the colon. The aim of this study was to examine the effect of raw potato starch alone and in combination with wheat bran on tumor development and precancer events in a rat model of colorectal cancer. Methods: Three groups of rats received either a low-RS/low-fiber ('basic') diet, the basic diet containing raw potato starch as 20% of carbohydrate content, or the potato starch diet plus 10% of 'wheat bran' fiber. Epithelial proliferation, aberrant crypt foci (ACF), and tumors were measured 6 and 20 weeks after a 10-week course of dimethylhydrazine. Results: Rats on the potato starch diet had tumors more frequently and had larger tumors than rats consuming the wheat bran or basic diets. Parallel effects on the density of ACF were found 6 weeks after the carcinogen. Although epithelial proliferation was significantly enhanced by potato starch compared with the basic diet, the addition of wheat bran did not suppress this enhancement. Conclusions: This type-2 RS enhances epithelial proliferation, ACF density, and tumor formation. The addition of wheat bran to an RS-containing diet suppresses tumorigenesis, acting on events responsible for the formation of ACF but not the events controlling the hyperproliferative phase.