We report a case of hydrocele with unusually shaped bone augmentation in the maxillary sinus, which occurred in a 62-year-old man. Computed tomography showed a cystic lesion arising around the apex of the upper left first molar in the left maxillary sinus, surrounded by an ossified mass. The clinical diagnosis was radicular cyst of the upper left first molar and a tumor in the left maxillary sinus. Under general anesthesia, we removed the tumor via the canine fossa and extracted the upper left first molar. The pathological diagnosis was hydrocele, and reactive bone augmentation consisted mainly of compact bone and partially of spongeous bone. As of 4 years after operation, there is no evidence of recurrence.
