The purpose of the present study was to compare the effect of sucrose and cornstarch on colon and liver carcinogenesis induced by 0.02% of the food-borne carcinogen 2-amino-3-methylimidazo [4,5-f]quinoline (IQ) in the feed. Male F344 rats were allocated to four groups. Two groups were fed diets high in either cornstarch (68%) or sucrose (34% sucrose/34% cornstarch) and were initiated with IQ. The remaining two groups received the same two diets but did not receive any IQ. In both liver and colon, administration of IQ resulted in a higher level of DNA adducts. In animals not dosed with IQ, sucrose increased the adduct level in both organs but to a lower level than IQ. However, simultaneous administration of IQ and sucrose did not further increase the adduct level. Both IQ and sucrose increased the expression of the DNA-repair enzyme ERCC1 in the liver. In the colon, the number of large and medium aberrant crypt foci (ACF) of the group fed IQ and cornstarch was significantly higher than that in the other groups. There was no statistically significant difference in any tumour incidence in IQ dosed-animals fed either cornstarch or sucrose. In conclusion, no difference in effect on liver carcinogenesis was seen between sucrose and cornstarch-based diets, however, the number of tumours per animal tended to be slightly higher in the rats fed cornstarch (P=0.08). Cornstarch enhanced ACF development induced by IQ when compared to sucrose whereas due to a low intestinal tumour incidence no correlation to diet and tumour incidence could be established.