OBJECTIVE Immunohistochemical study of carcinoembryonic antigen (CEA) distribution in inflammatory-regenerative and dysplastic changes of bowel mucosa. The relationship between the presentation of CEA and the intensity of inflammatory-regenerative and dysplastic processes in the flat colon mucosa was also examined. METHODS Biopsy specimens from 105 patients were examined: 45 classified as inflammatory-regenerative and 60 as dysplastic (21 mild, 23 moderate, and 16 severe dysplasia). The expression of CEA was assessed on the basis of location, quantity, and intensity of CEA immunostaining, by counting antigen-positive cells, and the presence of antigen in the lumen of crypts and glycocalyx on the surface of the mucosa. Using a semiquantitative method, antigen staining intensity was defined as weak, moderate, and intense. RESULTS The quantity of CEA in columnar, goblet mucous, and undifferentiated cells, as well as in the glycocalyx on the surface of the mucosa and in the crypts' content increased proportionally with the intensity of epithelial dysplasia. In inflammatory-regenerative lesions, CEA was located in the apical and supranuclear part of the cytoplasm, and in the dysplastic mucosa, in lateral and basal parts of the cells. CONCLUSIONS The quantity of immunohistochemically demonstrable CEA and its intracellular distribution changed in the colon mucosa during the transition from regenerative to dysplastic epithelial lesions. The intensity of CEA expression was closely related to the intensity of dysplastic lesion.