A 22 year old male patient was referred by his dentist to a department of Oral and Maxillofacial Surgery because of recurrent swelling of the oral mucosa of unknown origin in region 17. One year before, molar 17, although clinically sound but showing an apical radiolucency, was endodontically treated. Eight months ago, molar 17 was removed because of recurrent complaints. Clinical examination revealed a fistula with purulent drainage in region 17. Radiographic examination revealed the cause, viz. a follicular cyst originating from the right wisdom tooth located in the maxillary sinus. Thus, the persisting complaints in region 17 were not related to molar 17, but related to a chronically inflamed follicular cyst originating from an impacted third molar.