The close relationship between bowel cancer and other non-infective diseases of the bowel, such as benign tumor, divert ocular disease, and appendicitis, indicates that these conditions may have a common or related etiology. Their close association with the refined diet characteristic of economic development suggests that the removal of dietary fiber may be a causative factor. These diseases are all rare in every community examined which exists on a high residue diet, and common in every country where a low residue diet has been adopted. Dietary fiber has been shown to regulate the speed of transit, bulk, and consistency of stools, and together with other dietary factors is probably also responsible for the changes which have been demonstrated in the bacterial flora of feces. It seems likely that carcinogens produced by the action of an abnormal bacterial flora when held for a prolonged period in a concentrated form in contact with the bowel mucosa may account for the high incidence of these diseases in economically developed countries.