The transitional mucosa immediately adjacent to large bowel carcinoma was examined in 13 patients by scanning electron microscopy (SEM), and this was correlated with transmission electron microscopy (TEM) and light microscopy (LM). Marked abnormalities were present in this mucosa in 9/13 patients with distortion and loss of the normal architecture. Examination of adenomas showed abnormalities also to be present adjacent to the neoplastic mucosa. In all patients, the transitional mucosa was compared with two pieces of normal mucosa; in one patient, typical changes of transitional mucosa were found in this region. In the remaining patients, normal mucosa by SEM was found to consist to large deep furrows (primary crypts) as well as the crypts of Lieberkuhn (secondary crypts). The secondary crypts structure showed marked individual variation between patients but one type was present in nine patients. It is uncertain whether this is a pattern with a high incidence in the general population, or whether patients with this type of mucosa are particularly predisposed to large bowel carcinoma. SEM may, therefore, play a part in identifying patients particularly likely to develop large bowel carcinoma.