Colorectal carcinoma is becoming the most common form of visceral cancer in Western populations. A fat-related dietary factor is implicated in its pathogenesis, and evidence in man suggests that this factor may be cholesterol. Dietary cholesterol is co-carcinogenic in animals with colon cancer, and there is indirect evidence for a similar role in man. It is proposed that prolonged exposure to dietary cholesterol is co-carcinogenic for human colon cancer in that it facilitates the development, growth, and spread of this disease.