Cytokeratin (CK) expression was studied in 22 sinonasal inverted papillomas. Columnar (respiratory) epithelium in inverted papillomas abundantly expressed CK7, CK8, CK18, and CK19. Immunoreactivity for CK5/14 and CK17 was found in basal and parabasal/suprabasal cells. Transitional (cuboidal) and squamous epithelium in inverted papillomas comparably expressed CK7, CK8, CK18, and CK19. In addition CK13 was found in subluminal and surface cells. Immunoreactivity for CK5/14 and CK17 involved all layers of the epithelium. In nonpapillomatous nasal mucosa adjacent to inverted papillomas, CK expression in columnar (respiratory) epithelium exactly matched the findings in inverted papillomas. Transitional (cuboidal) and squamous epithelium in nonpaillomatous mucosa were negative for CK7, CK8, CK18, and CK19. CK13 was expressed in subluminal and surface cells. Immunoreactivity for CK5/14 and CK17 was restricted to basal and parabasal/suprabasal cells. Conclusively, transitional (cuboidal) and squamous epithelium in inverted papillomas but not in the adjacent mucosa coexpress CKs typical for columnar and squamous differentiation.