We investigated the effects of a high meat mixed Western diet and a nonmeat diet, representing the dietary pattern of high and low risk areas for colon cancer, respectively, on fecal microflora dn on bile acid and neutral sterol patterns in man. The total anaerobic microflora as well as the count of Bacteroides, Bifidobacterium, Peptococcus, and anaerobic Lactobacillus were significantly higher during the period of consumption of a high meat mixed Western diet comparted with the nonmeat-diet consumption period. The difference in total fecal bile acid excretion was not significant between the two dietary periods. Fecal excretion of microbially modified bile acids and neutral sterols was decreased when subjects eating a high meat diet transferred to a nonmeat diet. These results support the fact that diet plays a modifying role on the composition of intestinal microflora, bile acids, and neutral sterols.