To clarify the process to squamous metaplasia, we investigated ultrastructural features of the salivary ducts. And immunohistochemical staining was performed to determine the expression of cytoskeletal and intercellular adhesion molecules associated with morphological alterations of the salivary ducts. This study comprised 117 patients with chronic sialadenitis. The salivary ducts of the patients were microscopically classified into the following three groups: dilated ducts; hyperplastic ducts; and squamous metaplastic ducts. Squamous metaplastic ducts occurred in 34 of the 117 patients (29.0%). Immunohistochemical staining was performed in 25 patients with sialadenitis in whom the histological specimens were not decalcified at the time of preparation. Histopathological and ultrastructural analyses of the hyperplastic ducts showed hyperplasia of cuboidal basal cell-like cells with relatively high nucleus/cytoplasmic ratio. Immunohistochemically,hyperplastic basal cell-like cells were positive for p63, CK34βE12, CK5/6, CK19, and Ecadherin, thus reflecting basal cell characteristics. In immature squamous metaplastic ducts, positive reaction for CK18, CK5/6 and E-cadherin were attenuated. In mature squamous metaplastic ducts, CK13 appeared instead of E-cadherin and β-catenin. This study showed that dilation of ducts was due to abnormal physical irritation and that chronic irritation causes basal cell hyperplasia as a biological adaptive response. Basal cell hyperplasia and squamous epithelium characteristics were also demonstrated by ultrastructural analysis by using electron microscopy. These changes are also supported by the fact that cytokeratin in ductal epithelial cells undergo transition from a glandular epithelial type to mucosal type. These results suggest that basal cell hyperplasia is a squamous metaplastic process.