Purpose: This investigation evaluated the effectiveness of positron emission tomography with 2-[F-18]fluoro-2-deoxy-D-glucose (PET-FDG) in assessing residual tumor or tumor recurrence in postradiation nasopharyngeal carcinoma (NPC) patients.Procedures: Forty-six patients with histologically proven NPC who received radiotherapy were included. PET-FDG images were analyzed by a semiquantitative method, metabolic ratio (tumor to cerebellum ratio).Results: The overall sensitivity and specificity of PET-FDG to detect residual tumors and recurrent lesions in the postradiation patients were 80% (12/15) and 87% (27/31), respectively. In patients with PET-FDG 6 months after radiation therapy, the sensitivity and specificity raised to 92% (11/12) and 100% (20/20), respectively.Conclusions: PET-FDG is effective in the evaluation of NPC treated with radiation. The optimal timing in assessing residual tumor or tumor recurrence in postradiation patients should be 6 months or later.