The human evidence that dietary fiber prevents the development of colon cancer has been reviewed. The correlational studies are consistent with a protective effect in 61.9 per cent of reports. However, these studies are all retrospective and largely uncontrolled. The case-control studies provide evidence of a protective effect in only 48 per cent of reports. It is of additional concern that some human studies have found an association between tumor enhancement and some fiber-containing foods. Whereas this is not sufficient evidence to implicate dietary fiber as a promoter of human colon cancer, it does perhaps argue for a more conservative approach to recommending high-fiber diets as a means of cancer prevention. The animal data show that different sources of dietary fiber produce markedly different effects on colon carcinogenesis. Although some fibers exhibit protective properties, others clearly promote tumor development. The mechanisms behind these opposing actions require further investigation. However, one thing is clear and that is that dietary fibers do modulate the carcinogenic process and as such provide a valuable tool for probing the mechanisms and stages of colon tumor development. Dietary fiber appears to play a major role in the regulation of normal intestinal function and in the maintenance of a healthy intestinal mucosa. Although there is some evidence that a fiber-deficient diet predisposes to colon carcinogenesis, it is still not known whether an increase in fiber consumption will prevent the development of colon cancer. This is further complicated by not knowing what constitutes a normal level of fiber intake. In the interim, physicians should perhaps advise their patients to consume a moderate diet that contains vegetables, fruits, and whole grains. This will provide a varied source of fiber-containing foods and if consumed in sufficient quantity will optimize intestinal transit and bulk according to individual needs. Isolated fiber supplements have not been shown to be effective in colon cancer prevention. Further attempts to be more specific about what type of fiber to recommend seem premature at this time. However, an intake in the range of 20 to 35 gm per day of dietary fiber from foods has recently been recommended by an Expert Panel.