This study was aimed to observe the histological changes of the edentulous and denture wearing alveolar ridge mucosa. The distribution of Langerhans cells was also observed to investigate the mucosal immune response by denture wearing. The mucosal tissues were obtained from of 12 cases of edentulous nondenture wearers(NDW), 7 cases of denture wearers(DW), and 12 cases of flabby tissues(FT). For the identification of Langerhans cells of the mucosal epithelia, the immunohistochemical stain for S-100 protein was applied. The results were as follows : 1. 7 cases among 12 cases of NDW showed hyperkeratosis, and 5 cases were covered by parakeratosis, whereas 3 cases among 7 cases of DW showed hyperkeratosis, and 4 cases showed parakeratosis on the mucosal epithelium. All cases of both DW and NDW demonstrated epithelial hyperplasia, except. 2 cases of DW, which showed epithelial atrophy. The content of glycogen in the epithelial layer showed the decrease in the group of DW. 2. Both NDW and DW showed the infiltration of chronic inflammatory cells. The collagen fibers tended to be arranged densely and irregularly in cases of denture wearing period more than 10 years. 3. FT showed variable epithelial changes from epithelial atrophy to marked hyperplasia, and the pattern of keratinization was also variable. The collagen fibers tended to be arranged irregularly. 4. The distribution of Langerhans cells showed the increase of 1.84-1.96 times in the group of DW compared with NDW group.