Deoxycholic acid (DCA) has been shown to promote proliferation of colonic carcinoma cells in manyfundamental studies. However, no large-scale prospective clinical study providing direct evidence for anassociation of DCA with progress of colorectal tumor development in humans has been reported to date. Toaddress this question, we conducted a two-step epidemiological study applying enzyme-linked immunosorbentassays to measure fecal cholic acid (CA) and DCA concentrations. Firstly, we compared bile acid concentrationsof fecal samples from 366 patients who had multiple colorectal tumors removed endoscopically (tumor group)with those from 24 controls without abnormality in their large intestine (control group). Secondly, the tumorgroup was followed-up to evaluate the association between fecal bile acid concentrations and recurrence ofcolorectal tumors four years later. Fecal DCA level in the tumor group were significantly higher than that in thecontrols, whereas there was no difference in CA levels between the two groups. In the tumor group, a subgroupwith high DCA level had higher recurrence risk of large adenomas (> 3 mm) four years later than the low DCAsubgroup (odds ratio:1.85, 95% confidence interval: 1.12-3.05). This trend was observed more strongly in the leftside colon. In conclusion, a high fecal DCA concentration may be a promoter of colorectal tumor enlargement.