Dermoid cysts are malformations that are rarely observed in the oral cavity. Histologically, they can be further classified as epidermoid, dermoid or teratoid. We report a case in which a 25-year-old man developed an epidermoid cyst presenting as a large sublingual swelling causing speech and swallowing difficulties. The differential diagnosis for dermoid cysts includes infections, tumours, mucous extravasation phenomena and abnormalities arising during embryonic development. In this case, aspiration of the cyst produced a keratin-containing liquid, which proved to be useful in preoperative diagnosis. The lesion was surgically excised using an intraoral approach. Microscopic examination revealed a dermoid cyst of the epidermoid type. After 12 months of follow up, the cyst had not recurred. This case shows that dermoid cysts may be successfully diagnosed and managed using a series of simple yet effective clinical manoeuvres.