The clinical and pathological study was performed in order to determine the histopathological and cytoimmunological characteristics of denture stomatitis. All specimens were biopsy materials from seventeen patients with denture stomatitis. Normal palatal mucosae from ten patients served as the control. In addition to the usual staining methods, naphtol AS-D chloroacetate esterase stain and peroxidase-antiperoxidase method were used to detect mast cells and plasma cells. Denture stomatitis could be divided into atrophic and hyperplastic types. The former showed a smooth and atrophic mucosa. The latter showed a large number of exophytic projections which were composed of marked acanthosis and submucosal fibrosis, and was further subdivided into granular and papillary subtype according to the size of projections. In the present study, there were six cases of the atrophic type, and eleven cases of the hyperplastic type (consisting of seven granular and four papillary subtypes). The hyperplastic type was more frequently observed in patients with partial dentures compared with complete dentures and was associated frequently with ill fitting of the denture base as well as agglutination of denture plaque. Cytoimmunological study revealed that there was a pronounced increase of plasma cells, especially IgG- and IgA-producing cells, and a moderate increase of lymphocytes as well as mast cells in both types of denture stomatitis. Mast cells were always noted in the area with marked plasma cell infiltration, suggesting an intimate relation between both cells. These findings suggest that the immunological reactions play some role in the pathogenesis of denture stomatitis.