To verify the popular belief that the mucosa of the colon remote from a carcinoma is normal, in a retrospective study colon mucosae from 30 patients were studied; 15 patients had colon carcinoma and the other 15 patients without any obvious tumor in their colons served as controls. All the patients having colon carcinoma showed definite abnormalities in the mucosa remote from the tumor. None of the 15 control patients showed such morphologic changes in the mucosal sections sampled at random. The mucous changes observed were: dilatation and distortion of the crypts with flattening of the lining cells, overcrowding of crypts with mucous cells and basophilic cells, lining of the crypt with eosinophilic surface epithelial cells, and focal cellular stratification in the crypts. These abnormalities were also consistently observed in the transitional mucosa adjacent to the tumor in all of the 15 patients with colon cancer. Histochemical studies for the detection of epithelial acidic mucosubstances showed that sialomucin predominated in the colon mucosa harboring a carcinoma irrespective of the location of the tumor, whereas colon mucosa from otherwise normal individuals and patients with noncarcinomatous diseases showed a predominance of sulfomucin. Therefore, mucosa of the colon harboring a carcinoma was conclusively demonstrated to be morphologically and histochemically abnormal. The significance of these abnormalities and their possible role in the de novo histogenesis of colon carcinoma are discussed.