Several alterations in carbohydrate antigen expression can occur in colon cancers. Modified structures such as extended LeX and LeY antigens could serve as cancer-associated antigens in the human colon, although none is specific only for colon cancer. Since LeX and LeY antigens are present in fetal tissues, but not in adult normal tissues, these antigens appear to be oncodevelopmental in nature. The expression in colon cancers of extended LeX and LeY antigens with internal fucosylation or terminal sialylation is considered rather cancer-specific since normal colonic mucosa does not express these antigens. Furthermore, these molecules may also be "markers" for premalignancy, since adenomatous polyps, but not hyperplastic polyps are capable of exhibiting these changes and antigenic expression often correlates with malignant potential in adenomatous polyps. The precise biochemical and molecular mechanisms for these alterations in LeX and LeY expression and their biological significance are not well understood at the present time. However, it is likely that the regulation of the glycosyltransferase enzymes responsible for the processes of polylactosamine elongation, fucosylation, and sialylation may be aberrant. Obviously further studies are needed to elucidate these mechanisms. However, it appears that the monoclonal antibodies directed against extended LeX and LeY antigens are a useful adjunct in the diagnosis of colonic neoplasia, and may also be helpful in elucidating the molecular and biochemical mechanisms involved in the adenoma-carcinoma sequence.