AbstractThe effect of dietaryLaminaria angustata(brown seaweed) on azoxymethane (AOM)‐induced intestinal carcinogenesis was studied in male F344 rats. Five‐week old rats were fed semipurified diets containing 0 and 10% seaweed. When the rats were 7 weeks old, all except the vehicle‐treated groups received weekly subcutaneous injections of AOM in normal saline for two weeks (20 mg/kg body wt/week). All animals were fed the experimental diets until the termination of the experiment, which was 28 weeks after the last AOM injection. The incidence (percent of animals with tumors) and multiplicity (tumors/ animal) of small intestinal tumors did not differ significantly between the control and seaweed groups. The incidence and multiplicity of colon adenomas along with the size of colon tumors were increased in rats fed the seaweed diet compared with those fed the control diet. Dietary seaweed had no major effect on the concentration of fecal bile acids; however, the concentration of fecal cholesterol and total neutral sterols was decreased in the seaweed group. These results suggest that dietary seaweed increases the risk for colon tumors.