The morphological and histochemical features of colonic mucosa adjacent to 142 adenomas and 31 hyperplastic polyps were studied. Three predominant patterns were identified: (1) normal mucosa, showing normal histological architecture and secretion of sulphomucins; (2) N+ type, histologically normal mucosa with predominance of sialomucins; (3) transitional mucosa; hyperplastic mucosa secreting sialomucins. Hyperplastic changes were observed in the immediate neighbourhood or at the base of adenomas and were more frequent and extensive near large adenomas than around smaller lesions. Sialomucins were often predominant in the mucosa adjacent to large adenomas, but N+ type mucosa was also seen near minute adenomas and hyperplastic polyps and remote from polypoid lesions. Moreover, both hyperplastic and secretory changes were more frequent in the left colon than in the right. These findings seem to suggest that mucosal hyperplasia more likely represents a local change, parallel with or secondary to tumour development rather than a pre-adenomatous lesion. Secretory modifications are widespread and may result from the action of various factors among which carcinogens cannot be excluded.