A 49-year-old female developed a painful dysesthesia in the left forehead 10 years after having become aware of a subcutaneous mass accompanied with mild tenderness. Her medical history was unremarkable. Neurological examination showed normal findings with intact facial nerve function. Blood examination was normal. Neuroimaging revealed a cystic bony mass and an adjacent patchy diploic lesion in the left temporal bone. She underwent total tumor resection by frontotemporal craniotomy. Intraoperatively, a bony defect was noted in the lower surface of the patchy lesion communicating with the diploic venous system. A part of the dura mater was invaginated in the bony defect. The patchy lesion was found to communicate with the bony cyst through the diploic structure. Histological appearance of the bony cyst was consistent with the fibrous dysplasia accompanying cyst degeneration. Histological findings of the patchy lesion were identical to those of the bony cyst except for cyst degeneration. We assumed that the diploic venous system and the meningeal vessel might have been associated with the etiology of cyst degeneration in the present case of fibrous dysplasia.