Dermoid cysts commonly occur in the anus and ovaries. The occurrence rate of dermoid cysts in the head and neck area and in the oral cavity is 6.9% and 1.6%, respectively. Dermoid cysts are rarely gigantic and most of them develop in the sublingual area. We report a rare case of a giant dermoid cyst that advanced to the submental area in a 51-year-old woman who visited our hospital with the chief complaint of cervical swelling. Magnetic resonance imaging （MRI） and contrast-enhanced computed tomography （CT） revealed a monocystic lesion that was narrowed at the midline, had symmetrically developed into the submandibular area, and was located between the geniohyoid and the platysma muscle. The size of the lesion was 112×50×30mm. It was clinically diagnosed as a submental dermoid cyst and was enucleated by an extraoral approach under general anesthesia. The cyst wall was thick and its adhesion to the surrounding tissue was unclear, which permitted easy detachment. The histopathological diagnosis was a dermoid cyst. It was reported that most giant dermoid cysts in the oral cavity are of the sublingual type. Giant dermoid cysts also have a risk of malignant transformation. In the present case, there was no evidence of recurrence 5 years after the surgery.
