Purpose: To investigate the healing pattern of the mucous membrane after tooth extraction necessitated by periodontal disease in the maxillary sinus. Methods: One hundred and three patients with 119 maxillary sinuses were investigated. Before implant placement, cone-beam computed tomography (CT) scanning was performed. The causes of extraction, the time elapsed since extraction, smoking, periodontal disease in adjacent teeth, and gender were recorded. In addition, the thickness of the mucous membrane of the maxillary sinus and the height of residual alveolar bone at the extracted area were calculated from CT images. Results: The thickness of the mucous membrane in the periodontal disease group ($3.05{pm}2.71;mm$) was greater than that of the pulp disease group ($1.92{pm}1.78;mm$) and the tooth fracture group ($1.35{pm}0.55;mm$; P<0.05). The causes of extraction, the time elapsed since extraction, and gender had relationships with a thickening of the mucous membrane of the maxillary sinus (P<0.05). In contrast, the height of the residual alveolar bone at the extracted area, periodontal disease in adjacent teeth, and smoking did not show any relation to the thickening of the mucous membrane of the maxillary sinus. Conclusions: The present study revealed distinct differences in healing patterns according to the causes of extraction in the maxillary sinus, especially periodontal disease, which resulted in more severe thickening of the mucous membrane.