A large-sized dermoid cyst of the floor of mouth is rare. We report an 18-year-old woman with a 9.0×4.8 cm cyst in the floor of mouth. Magnetic resonance imaging (MRI) revealed it to be of the sublingual type in the classification of Bergmann-Hagisaki, and we could excise it by employing an intra-oral approach. The symptoms subsequently disappeared. Histopathologically, it was diagnosed as a dermoid cyst. Recurrence has not been observed for 3 months after surgical treatment.
