We have observed and defined morphometrically and histochemically groups of undifferentiated columnar cells within the surface epithelium in colorectal mucosa. They were present within both non-polypoid and polypoid mucosa in familial adenomatous polyposis, and within non-hereditary adenomatous polyps of the colon and rectum. The cells show some evidence of proliferative activity and appear similar to cells previously described in the stomach which were proposed as precursors to type 3 sulphomucin-secreting intestinal metaplasia in atrophic gastritis. To our knowledge, these observations have not been previously described. It is possible that the cells represent the cellular basis of the shift in the proliferative zone from the normal site at the crypt base to the colorectal mucosal surface, which is known to precede adenomatous polyp formation. The cells may therefore be involved in the early stages of colorectal adenoma formation.