The Apc1638N mouse model, which carries a targeted mutant allele within the adenomatous polyposis (Apc) gene and develops intestinal tumours spontaneously, predominantly in the small bowel, was used to investigate the effects of two potential chemopreventive agents, aspirin and alpha-amylase resistant starch (RS). Heterozygous Apc+/Apc1638N mice were fed semi-purified diets rich in animal fat, animal proteins and sucrose and low in dietary fibre (Western style diets) from approximately 6 weeks up to 6 months of age. Two of the diets contained aspirin (300 mg/kg diet) and two RS (1:1 mixture of raw potato starch: Hylon VII at 200 g/kg diet) in a 2 x 2 factorial design. A fifth treatment group were fed a conventional rodent chow diet. The mice fed the Western style diets became almost three times as fat as the chow-fed mice but this did not affect tumour yield. Treatment with RS resulted in significantly more intestinal tumours whereas aspirin alone had no effect. However, there was a significant aspirin x RS interaction, which suggests that aspirin could prevent the small intestine tumour-enhancing effects of RS in this Apc-driven tumorigenesis model. The possibility that large amounts of purified forms of resistant starch may have adverse effects within the small bowel is a novel observation that requires further investigation since greater intakes of starchy foods (and of RS) are being encouraged as a public health measure in compensation for reduced dietary fat intake. However, it remains possible that any increased risk is restricted to carriers of germline mutations in APC.