Dietary factors are considered important environmental risk determinants for colorectal cancer development. Epidemiological studies have shown that a high fat (or meat) intake is associated positively and a high starch, fibre (non-starch polysaccharide), vegetable and fruit intake negatively with colorectal cancer incidence. One mechanism by which these effects are possibly exerted is through the metabolism of secondary bile acids. Secondary bile acids are formed after enzymatic deconjugation and dehydroxylation of primary bile acids in the large bowel by anaerobic bacteria. It has been shown that these compounds can have tumour-promoting capacities in animal experiments. In epidemiological studies, colonic cancer risk is related to the faecal bile acid concentration. In serum and bile of patients with colonic adenomas, more deoxycholic acid was detected than in healthy controls. Secondary bile acids are toxic to several cell systems at physiological concentrations. The exact mechanism by which these amphiphilic molecules exert their action is not well understood. It might act through membrane damage, intracellular mitochondrial action or genotoxic effects. So far the evidence that bile acids are involved in colonic carcinogenesis is largely circumstantial. It is, however, well accepted that environmental factors, such as dietary habits influence genetic susceptibility. Bile acids could play a promoting role in this process.