A previous report of members of our group demonstrated, in most adenocarcinomas and some adenomatous polyps with areas of malignant transformation, the presence of glycogen in cells that failed to show the synthesis of ordinary mucosubstances. The presence of glycogen in non-differentiated epithelium of the digestive tract of embryos, where it precedes ordinary mucus secretion, has lead some authors to suggest that the changes to glycogen secretion that occurs in malignant colonic cells, reflects the general regression to a less differentiated level of structure and function. It is well established that patients with carcinomas of the colon classified as less differentiated or higher grade tumors have significantly less chance of cure and consequently a worse prognosis than patients with lower grade tumors. The present ultrastructural study of the morphological differentiation characteristics of the glycogen containing cells previously encountered, revealed a strong association of glycogen to undifferentiated cells of the colonic adenocarcinomas and precancerous polypoid lesions studied. This undoubtedly seems to encourage the testing for the presence of glycogen in routine procedures as a possible marker of diagnostic and prognostic significance.