Carcinoma of the colon and rectum is the most common serious type of cancer found in the United States and is second only to lung cancer among causes of death from cancer. Its cause is unknown but several environmental factors-especially low bulk, high fat diets-seem to predispose to its development. The disease is readily treatable by surgical operation if it is diagnosed early. Radiation and chemotherapy may offer some additional benefit in treating advanced disease but the response to all forms of therapy is disappointing in patients in whom disease has spread beyond the bowel wall. Colorectal cancer appears to be a very slowly progressive disease with a long asymptomatic period providing an ideal opportunity for diagnosis at an early treatable stage. Both proctosigmoidoscopy and screening specimens of stool for occult blood have been shown to be effective methods for identifying it before symptoms develop. These procedures should be done routinely in all patients over 40 years old and especially in those patients who have other risk factors such as positive family histories or hereditary conditions known to predispose to colorectal cancer.