AbstractDiet‐induced changes in fecal excretion of secondary bile acids, certain neutral sterols, and bacterial enzyme activities are known to play a role in colon cancer development. Dietary fish oil (FO) has been implicated as a protective agent in colon carcinogenesis. In the present study, the effects of FO and corn oil (CO) on these fecal parameters were investigated in 24 healthy volunteers consuming a low‐or a high‐fat diet (30% or 50% of energy derived from fat). After four weeks of FO or CO supplementation (4.4 g of n‐3 fatty acids/day), no significant differences were noted for fecal activities ofβ‐glucuronidase,β‐glucosidase, and sulfatase, nor was fecal bile acid excretion significantly affected by FO or CO consumption. However, daily excretion of the putative colon carcinogen 4‐cholesten‐3‐one was significantly lower in the FO than in the CO period during low‐and high‐fat experiments. This may be another biochemical mechanism by which FO exerts its protective effect on colon cancer development.