Tehran University of Medical Sciences Frontiers in Dentistry 2676-296X 10 6 2013 12 15 562 8 Omid Mahdavi Assistant Professor, Department of Oral and Maxillofacial Medicine, School of Dentistry, Shahid Sadoughi University of Medical Sciences, Yazd, Iran. Najmehalsadat Boostani Resident, Department of Anesthesiology, School of Medicine, Shahid Sadoughi University of Medical Sciences, Yazd, Iran. Sharareh Karimi Assistant Professor, Department of Oral and Maxillofacial Medicine, School of Dentistry, Kurdistan University of Medical Sciences, Sanandaj, Iran. Adel Tabesh Resident, Department of Oral and Maxillofacial Medicine, School of Dentistry, Shahid Sadoughi University of Medical Sciences, Yazd, Iran. 2015 10 16 Sinonasal undifferentiated carcinoma is an extremely rare malignancy of the paranasal sinuses and nasal cavity. It is of unknown etiology, and occurs more commonly in the elderly men, with a routinely shown aggressive behavior and poor prognosis for survival. Radiographically, it looks like severe osteomyelitis. Histopathologic study is essential to confirm diagnosis, and the undifferentiated histologic appearance often necessitates immunohistochemical studies for differentiation from other high-grade neoplasms. We present an 83-year-old man complaining of pain and unilateral swelling on the left side of the face due to a rare malignant tumor of maxillary sinus origin, a sinonasal undifferentiated carcinoma. He underwent hemimaxillectomy and radiotherapy, but refused chemotherapy. Maxillary sinus malignancy may be presented with unspecific symptoms mimicking sinusitis or dental pain. Coming across such symptoms, the physician or dentist must consider malignancies as well, and carry out medical and dental workups. https://fid.tums.ac.ir/index.php/fid/article/view/577 https://fid.tums.ac.ir/index.php/fid/article/download/577/552