A false-positive F-18 FDG PET scan caused by osteoradionecrosis and inflammation in patients with nasopharyngeal carcinoma (NPC) has been reported. The standard uptake values (SUVs) in these false-positive cases are always below 6 and decrease with time. This report is concerned with a false-positive result with extraordinarily high F-18 FDG uptake, which increased on the delayed 3-hour image. A 63-year-old man with NPC underwent surgical removal and radiotherapy (7400 cGy) in July 2003. In October 2003, the CT scan showed a large soft tissue mass over the nasopharynx. Tumor recurrence was suspected. F-18 FDG PET scan showed an FDG-avid lesion over the nasopharyngeal region with a maximum SUV of 28.8 at 1 hour that increased to 30.4 at 3 hours. Biopsy was performed and the histopathologic examination showed only necrotic tissue and no evidence of tumor cells. Radiation necrosis was diagnosed. No evidence of tumor recurrence was noted during the 10-month follow-up period.