The natural history of colorectal cancer. Opportunities for intervention. There is now a better understanding of the natural history of colorectal cancer, which has provided a basis for intervention to influence outcome. The possible interventions include earlier detection of colorectal cancer, removal of premalignant adenomas, demonstration of the mucosal field defect that precedes neoplasia to evaluate baseline risk and its change with dietary modification, and identification of inherited and dietary risk factors. Five controlled trials evaluating early detection of colorectal cancer with fecal occult blood testing have enrolled more than 309,000 patients. Early stage cancers with improved survival has been observed, but data on mortality reduction have not as yet been reported. Studies of patients with adenomas have demonstrated high synchronous and metachronous rates as a basis for complete colon evaluation initially and a surveillance follow-up program. Hyperproliferation and lack of normal differentiation have been observed as a field defect in the colon preceding neoplasia. Inherited factors have recently been shown to be important in a larger proportion of individuals destined to develop colorectal adenomas and cancer. These observations of the natural history of colorectal cancer have provided new opportunities for the application of radiologic and endoscopic techniques in diagnosis and surveillance; each examination has its merit. Further research is needed to answer many critical questions that have been raised regarding the impact of these interventions. 