Small nodules are commonly seen in the rectal mucosa during sigmoidoscopy of patients over 40 years of age and are often noted in rectums excised for carcinoma. There is little information in the literature on the pathology of these small nodules. There is evidence that there are various histological types which are possibly of different clinical significance. This problem has been studied using a series of rectums removed surgically for carcinoma and a comparable series of rectums obtained at necropsy from subjects with no evidence of large bowel disease. The incidence, distribution, and histological structure of nodules more than 1 mm in diameter has been studied. The commonest histological types of lesion are found to be adenomas and metaplastic nodules. The histological structure, mode of formation, natural history, and clinical significance of metaplastic nodules is considered in detail. It is concluded that metaplastic nodules are a normal finding in the rectal mucosa of subjects over 40 years old. It is suggested that they represent an aging change in the mucosa. They are easily distinguished histologically from adenomas and evidence is presented that they have no relation to them and are not liable to undergo malignant change.