The radicular cyst is the most common of the odontogenic cystic lesions. Treatment options include conventional endodontic therapy and surgical enucleation, with extraction of the offending tooth if necessary. The present case had root canal treatment for the upper left first molar one year ago because of constant left face swelling and pain. Radiographic examination revealed a large unilocular radiolucency involving the maxillary sinus and adjacent molars. During conventional endodontic therapy the patient had his upper left second molar extracted due to exacerbation of the offending tooth. The ensuing symptoms of acute maxillary sinusitis persisted for over two weeks. The patient was referred to the medical center for cyst enucleation via the transoral approach along with the extraction of his upper left first and third molars. Histopathological examination confirmed a radicular cyst with cavity lined with non-keratinized stratified squamous epithelium. There were no signs of recurrence nine months later after surgery.