The malignant potential of adenomas of the colon and rectum varies with size, histological type and grade of epithelial atypia. The adenomatous polyp is usually small and has a low malignant potential, whereas tumors with a villous structure are usually larger and have a much higher cancer rate. Severe atypia is more common in villous adenomas than in adenomatous polyps. Evidence is presented which suggests that most cancers of the colon and rectum have evolved through the polyp-cancer sequence although the majority of adenomas do not become cancerous during a normal adult life span. The slow evolution of the polyp-cancer sequence is stressed. The implications of the polyp-cancer sequence for the design of cancer prevention programmes and the study of the aetiology of large bowel cancer are discussed.