A case is reported of spontaneous bone formation on the maxillary sinus floor associated with the extraction socket of a maxillary impacted tooth. An impacted maxillary second premolar of a 20-year-old male had been pushed into the maxillary sinus during surgical extraction. The tooth was removed using the sinus elevation technique. After 5 months of healing, the space between the sinus floor and the socket was filled with new bone. Later, implant surgery was successfully carried out without any sinus augmentation. Osteogenic activity of sinus mucosa and the blood clot in the extraction socket beneath the elevated sinus would have been important factors in this spontaneous bone formation.