Positron emission tomography (PET) is a promising method for pretherapeutic assessment of spread of squamous cell carcinomas (SCC) in the head and neck. A 41-year-old man with a history of operated and irradiated SCC of the tongue presented 4 years later with symptoms and signs of mandibular osteoradionecrosis. No changes related to malignancy could be seen in panoramic radiographs or computed tomography scanning with contrast enhancement. Since a biopsy of the involved region was positive for SCC, a PET study with [18F]fluorodeoxyglucose (FDG) was performed. In dynamic PET images, intensive uptake of FDG was seen in a small area close to the right mental foramen. A hemimandibulectomy with reconstruction using a free vascularized graft from iliac crest was performed. In the resected specimen, histological examination showed a 1.2-mm focus of SCC in the soft tissue and bone around the mental foramen. These findings indicate that FDG-PET might be useful for presurgical evaluation of cancer recurrence in a previously irradiated mandible, especially if PET can accurately differentiate viable tumor tissue from radiation-induced fibrosis and inflammation.