The present study was undertaken to investigate certain dietary factors known to affect the development of colon cancer for their ability to modulate aberrant crypt foci (ACI;). Male Wistar rats were initiated with oral noses of dimethylhydrazine dihydrochloride (DMH-2HCl, 20 mg/kg body wt) once a week for to or 20 weeks. Throughout the study the animals were fed I) semisynthetic casein-based control diet, 2) control diet with 20% lard, 3) control diet with 20% lard and 20% dietary fiber, or 4) control diet where most of the carbohydrate pool was substituted with sucrose and dextrin. The composition of the different diets was designed to achieve equivalent intakes of essential nutrients. Animals were killed after 10, 20, and 31 weeks. The study showed a pronounced effect of dietary composition on the development of DMH-induced ACF. The diet high in sucrose and dextrin caused a statistically significant increase (p less than or equal to 0.05) in the total number of ACF and number of small and medium ACF. Adding lard to the standard diet did not cause an increase in ACF, bur if the dietary fiber was added to the high-fat diet, a statistically significant reduction (p less than or equal to 0.05) in the total number of ACF and number of small and medium ACF was observed. The values of large and extra-large foci reflected the same effect of diets on ACF. The results indicate that tumors in the group fed the diet high in refined carbohydrates were more prominent and occurred with a higher incidence. However, the difference is based on few tumors and is not statistically significant. Our results do not show that the number of ACF and crypt multiplicity are conclusively predictive for tumor outcome with the present protocol, which did not include parameters to differentiate between ACF at the cellular level.