Fecal bile acids were analyzed by gas chromatography in 10 patients with colon cancer, 25 patients with colon polyp and 10 patients with peptic ulcer. On admission total bile acid excretion in patients with colon cancer was significantly higher than in patients with peptic ulcer (P less than 0.01). Also, concentration of cholic acid, chenodeoxycholic acid and primary bile acids in patients with colon cancer were significantly higher than those in patients with peptic ulcer (P less than 0.05, P less than 0.025) and in patients with colon polyp (P less than 0.005). Primary bile acids in patients with colon cancer significantly decreased under ward conditions after admission (P less than 0.05). Total bile acids in patients with colon cancer decreased after admission though there was no significant difference. It is of considerable interest that the fecal bile acid composition of colon cancer patients was different from that of colon polyp and peptic ulcer patients. Fecal bile acids may be involved in the pathogenesis of colon cancer.