Numerous lifestyle factors have been implicated in colorectal carcinogenesis. These include diet, inadequate physical activity, obesity, alcohol consumption, and smoking. Epidemiologic studies, animal experiments, and randomized clinical trials have shown that dietary factors can influence all stages of colorectal carcinogenesis, from cell proliferation to transformation to cancer. Defining the precise role of diet and other lifestyle factors in colorectal carcinogenesis may require the elucidation of genetic susceptibility and genetic-environmental interactions. Despite the preoccupation with nutrition by the public and the media in the United States, trends in food consumption have not been favorable. The average U.S. diet is still too high in calories and fat and too low in fiber, cereals, fruits, and vegetables. Dietary modifications along with secondary prevention measures may have a major impact on reducing the mortality from colorectal cancer.