We report a case of inverted ductal papilloma (IDP) and mucous cyst on the lower lip. A 39-year-old man was referred to our department in August 2017 for a persistent mass on the lower lip after biting it by mistake in June. The mass was temporarily reduced at the hospital, but it began to re-swell in March 2019. We noted a hemisphere-shaped soft-elastic mass with a diameter of 10 mm in the middle of the lower lip mucosa. The mass showed a low signal on T1-enhanced contrast magnetic resonance imaging, and a mildly high signal on T2-enhanced imaging. Ultrasound revealed that the mass was internally anechoic. A clinical diagnosis of soft-tissue cyst was made, and removal of the mass was performed. The cysts were filled with a milky fluid and were adherent to the surrounding tissue. Histopathologically the ductal epithelium had proliferated, inverting into the papillary. We observed granulation tissue surrounded by a mucous cyst. A histopathological diagnosis of IDP and mucous cyst was made. No recurrence was noted one year after surgical removal of the mass.
