Rectal biopsies and fecal collections were obtained from a consecutive series of 34 outpatients prior to colonoscopy at a gastroenterology clinic. Subsequently, 14 were found to have no colonic pathology, 13 had adenomatous polyps, (3 of those had a previous history of colon cancer) and 7 were diagnosed with colon cancer. In confirmation of earlier studies the tritiated thymidine labelling index was higher in patients with tumors than in those without pathology (7.9% vs. 5.8% with P = 0.06). The patients with colonic tumors also had significantly higher levels of deoxycholic acid (P = 0.01) and lithocholic acid (P = 0.005) in the aqueous extract of their feces. This study shows that these biochemical measures may indicate colon cancer risk.