Dietary supplements of calcium, vitamins A, C, and E, carotenoids, and omega-3 fatty acids can reduce the yield of experimental cancers in animals and reverse the pattern of abnormal epithelial proliferation in animals and humans. Epidemiological studies indicate that diets containing high amounts of these agents convey a protective effect against the development of colon cancer. Moreover, regular aspirin use in humans appears to reduce the risk of colon cancer and sulindac causes regression of polyps in patients with familial polyposis. These agents are promising for the prevention of human colorectal cancer, but their efficacy has not yet been shown in prospective, controlled trials. Thus, although it is tempting to speculate that in the future we may treat our patients who have a predisposition to colon polyps and cancer, or even healthy people at average risk, with such ordinary supplements as calcium, vitamins, fish oil, or aspirin, such advice at this time is premature.