Aberrant crypt foci (ACF) have been identified in the methylene-blue stained mucosa of the human colon. Some lines of evidence suggest that ACF may be precursors of colon cancer. The objective of the present study was to establish morphological criteria able to define and classify ACF in histological sections. Twenty-four colectomy specimens were collected after operation for colorectal cancer and fixed in 10% formalin. Strips of grossly normal mucosa were stained in a 0.2% solution of methylene blue in saline for 5-10 min. The strips were measured, put on a glass slide and observed under a light microscope at x25. One hundred and fourteen ACF identified by topology were sectioned parallel to the muscularis mucosae. Eighty-four ACF were evident at histological examination and could be classified into three main groups: group A (61 ACF, 72.6%) including foci whose epithelial cells had regular nuclei, with only mild or focal crowding but no stratification, no mucin depletion and no dysplasia; group B (16 ACF, 19.1%), in which features of hyperplasia were evident; and group C (seven ACF, 8.3%) including foci with enlarged, crowded and stratified nuclei, mucin depletion, frequent mitoses, and evident dysplasia, diffuse or focal (mild in five cases, moderate in two) representing microadenomas. Finally, hyperplastic foci were significantly larger than foci of group A and C. Group B ACF were also more frequent in the rectum than in the colon. In conclusion, selected histological features allow the definition of groups of ACF, which may represent sequential steps in the development of human colorectal tumours.