Aetiology of adenoma--carcinoma sequence in large bowel.

A hypothesis, based on epidemiological and histopathological data, is proposed to account for the aetiology of colorectal adenomas and carcinomas. Adenomas are initially caused by an environmental agent which acts on the colorectal cells of adenoma-prone persons, a further environmental agent causes small adenomas to grow, and a carcinogen causes malignancy in a high proportion of large adenomas.