A 27-year-old man sought health care, complaining of discomfort and drainage of purulent secretion in the retromolar region of the mandible. Clinical examination revealed a slight volume increase of the left buccal bone plate, effacement of vestibule fundus, drainage of white/yellow content from gingival sulcus of tooth 37, and moderate painful symptoms on palpation. Radiographic examination revealed a well-delimited, multiloculated, radiolucent image, extending from the periapical region of tooth 37 to the middle portion of the mandible ramus. Diagnostic hypotheses were keratocyst and ameloblastoma. Aspiration puncture revealed reddish content. An incisional biopsy was performed, and the lesion was decompressed with a drain. Histopathological evaluation reported a lesion compatible with fusocellular injury of uncertain nature. Immunohistochemical evaluation showed positive CD34 (QBEnd10), Vimetine (VIM3B4), and low Ki67 antigen (KiS5) expression. A 3-month follow-up showed bone neoformation in the entire extension of the lesion. The final diagnosis was a secondarily infected cystic lesion. A 27-year-old man sought health care, complaining of discomfort and drainage of purulent secretion in the retromolar region of the mandible. Clinical examination revealed a slight volume increase of the left buccal bone plate, effacement of vestibule fundus, drainage of white/yellow content from gingival sulcus of tooth 37, and moderate painful symptoms on palpation. Radiographic examination revealed a well-delimited, multiloculated, radiolucent image, extending from the periapical region of tooth 37 to the middle portion of the mandible ramus. Diagnostic hypotheses were keratocyst and ameloblastoma. Aspiration puncture revealed reddish content. An incisional biopsy was performed, and the lesion was decompressed with a drain. Histopathological evaluation reported a lesion compatible with fusocellular injury of uncertain nature. Immunohistochemical evaluation showed positive CD34 (QBEnd10), Vimetine (VIM3B4), and low Ki67 antigen (KiS5) expression. A 3-month follow-up showed bone neoformation in the entire extension of the lesion. The final diagnosis was a secondarily infected cystic lesion.