The risk of developing gastrointestinal cancer varies markedly between and within populations. These variations indicate that environmental factors, especially those associated with diet, are correlated strongly with the risk of developing intestinal tumours. In particular a diet high in animal fat and protein carries a high risk for colon cancer whereas the consumption of salted or pickled foods is associated with gastric cancer. Although many carcinogens have been identified in various food items none have been associated specifically with the incidence of a particular gastrointestinal cancer, not have any specific carcinogens been isolated from colonic or gastric contents. Mutagens, presumptive procarcinogens, have been detected in faecal and gastric samples, but, owing to problems with experimental design, have not been correlated with risk. As most common intestinal cancers arise in those segments of the intestine that are, or may be, most heavily colonised it has been hypothesised that these bacteria may form biologically active substrates in situ. In vitro studies have confirmed that intestinal bacteria can metabolise a wide range of substrates, such as steroids, bile acids and amino acids, to potential precarcinogens. Furthermore, the substrate for these reactions are generally present in higher concentration in the faeces of high risk colon cancer subjects, although similar data are not available for potential stomach cancer subjects.