Subperiosteal implants are generally used in the severely resorbed areas, and although dwindling in use, they have shown to be successful treatment options for the edentulous dental patients. We report a case of maxillary subperiosteal implantitis that caused sinusitis. A 59-year-old man was referred to our clinic in March 2003 with a chief complaint of pain and swelling in the bilateral upper molar region. He received a surgical operation for maxillary subperiosteal implant at another dental clinic in April 2001. CT scans demonstrated that the radiopacity of the right maxillary sinus extended to the ethmoid and frontal sinuses. Extensive surgical debridement was performed. The radiopacity of the right maxillary sinus in CT almost disappeared in a month. Two months later, the occlusal function was recovered using a new maxillary complete denture. The patient has had no symptoms and no occlusal disturbance by the denture for more than nine years.