Dermoid cysts are unusual neoplasms and can occur in every part of the human body. They represent only 6.9% of all dermoid cysts in the head and neck region; in the oral cavity, the incidence is low, approximately 1.6% of all dermoid cysts. Our aim is to present an unusual case of a large sublingual dermoid cyst with mandibular prognathism caused by cyst growth. We reported a case of a large sublingual dermoid cyst in an 8-year-old female patient. A bibliographic research from 1937 to 2013 is reviewed and we found only three cases of mandibular deformity, of which only one was a dermoid cyst of the floor of the mouth. Removal of dermoid cysts of the floor of the mouth should be completed as early as detected, especially in newborns and infants when osseous growth abnormalities could result if removal is delayed.