Immunohistochemical interpretation of early epithelial disorders of pyriform sinus. Expression of cytokeratins (CKs) was investigated immunohistochemically by use of monospecific monoclonal anti-CK antibodies in normal epithelia of pyriform sinus and epithelial lesions such as simple hyperplasia, different degrees of dysplasia, in situ carcinoma, and invasive carcinoma. In normal epithelium, strong expression of CK-19 was consistently observed only in the basal layer as basic CK, while expression of CK-13 showed a completely reverse pattern, being expressed only in suprabasal layers as stratification-related CK. Characteristic changes in expression pattern of these two CKs were observed in accordance with the degree of epithelial disorders and differentiation of carcinoma. Cytokeratin 1, as keratinization-associated CK, was observed only in keratinized cells of hyperplastic epithelia and well-differentiated carcinomas. These findings may be useful in evaluating epithelial disorders and classifying carcinomas more objectively, and may assist earlier detection of carcinoma when used with standard histologic techniques. 