Abstract: Objective To explore the clinical value of ¹⁸F-fluorodeoxyglucose positron emission tomography (¹⁸FDG-PET) in three-dimensional conformal radiotherapy (3D-CRT) for locally residual and recurrent nasopharyngeal carcinoma (NPC). Methods Between July 1998 and October 2000, 36 patients with recurrent or residual NPC confirmed by biopsy after initial radiotherapy underwent treatment with 3D-CRT. CT simulation was performed and ¹⁸FDG-PET carried out for delineating the gross tumor volume (GTV) in all the patients. The radiotherapy was designed by 3D treatment planning system and dose-volume histogram (DVH) was used to verify and optimize the radiation plan. Acute and late radiation injuries, survival rates and GTV delineated by CT and ¹⁸FDG-PET were observed in all the cases after the treatment. Results In comparison with CT simulation, ¹⁸FDG-PET delineated similar GTV in 36.1% (13/36) cases, larger and smaller GTV in 16.7% (6/36) and 47.2% (17/36) of the cases, respectively. After 3D-CRT, 97.2%, 75.0%, and 58.3% of the patients were free of evidence of 1-, 2-, and 3-year local failure, respectively, and the 1-, 2-, and 3-year survival rate of the patients were 94.4%, 86.1%, and 72.0%. The 1-, 2-, 3-year tumor-free survival rates were 85.7%, 76.6%, and 56.1% respectively. Acute xerostomia, nasopharyngitis and oropharyngitis were the most common complications in relation to the radiation, but all within acceptable range of grades 1 or 2. The lesions resulting from the radiation included hearing loss, trismus, dysphagia, chronic sinusitis, and cranial neuropathy, with the incidences of 5.6% (2/36), 25.0% (9/36), 8.3% (3/36), 33.3% (12/36), and 13.9% (5/36) respectively. Conclusions 3D-CRT combined with ¹⁸FDG-PET for delineation of GTV is effective and feasible in the treatment of locally residual or recurrent nasopharyngeal carcinoma after initial radiotherapy, producing good responses without serious complications.