A case of a large mucous cyst occurring in the buccal mucosa of a 27-year-old woman is reported. She visited the First Department of Oral and Maxillofacial Surgery, Kyushu Dental College Hospital because of a painless swelling on the left buccal mucosa. The lesion measured 19 × 26 mm and was a soft elastic mass covered with adipose tissue. The mass was located under the buccal mucosa of the left mandibular premolar region. The mass was not fixed to the surrounding tissue. We resected the mass under general anesthesia. A benign tumor was suspected. The mass had a thin outer capsule and was filled with fluid. We diagnosed the mass to be a mucous cyst during operation. The diagnosis was confirmed histopathologically. This was a rare case of mucous cyst with respect to the site of occurrence and size. At first, we strongly suspected that the mass was a benign tumor on the basis of the clinical and MM findings. Diagnosis was difficult because there were many similarities with other types of cysts and tumors in the affected region. Preoperative examinations such as computed tomography and ultrasonography besides MRI appeared to be necessary to evaluate the contents of the lesion.
