Because of the potential significance of secondary bile acids and cholesterol metabolites formed by the intestinal microflora in the pathogenesis of colon cancer, fecal microbial 7alpha-dehydroxylase and cholesterol dehydrogenase activities were assayed in colorectal cancer patients, patients with adenomatous polyps, asymptomatic controls, and normal controls. The mean 7alpha-dehydroxylase activity per 100 mg of dry feces per 2 hr incubation for colorectal cancer patients was significantly greater than either asymptomatic controls or normal controls. Patients with polyps had greater activity than controls, but did not differ significantly from the cancer patients or asymptomatic controls. The mean cholesterol dehydrogenase activity for colorectal cancer patients was significantly greater than controls. Patients with polyps also had greater activity than controls. No differences in the level of fecal beta-glucuronidase activity were found among the four risk groups. These data support the concept that patients with colorectal cancer are more able to convert primary bile acids and cholesterol to microbial products in colon contents than are controls. Such differences in the activity of intestinal microflora may serve as biochemical indicators that will reflect the enzymic activity of the fecal flora as well as the key intraluminal compounds in identifying populations at different risk for developing colorectal cancer.