Purpose: The aim of this study was to determine the incidence and malignant rate of incidental asymmetric palatine tonsillar uptake (ATU) on 18F-FDG PET/CT in various clinical indications and to evaluate the clinical and PET/CT findings suggesting malignancy. Materials and Methods: We retrospectively reviewed a total of 2,901 patients (58.4¡¾12.3 yrs, range 20~88 yrs, M:F=1,841:1,060) who underwent 18F-FDG PET/CT during an 1-year period with various indications except primary tonsillar cancer and lymphoma evaluation. On 18F-FDG PET/CT, metabolic abnormality of the palatine tonsil and cervical lymph node were visually assessed. ATU was defined as increased palatine tonsillar uptake with diffuse, focal, or irregular pattern compared to contralateral side. The incidence and malignant ratio of ATU were evaluated according to clinical and PET/CT findings. Results: Of 2,901 cases, 290 (10.0%) showed ATU. The incidence of ATU showed seasonal variation and was high in the winter (12.1%). Of 209 cases with ATU confirmed pathologically and/or clinically, five (2.4%) were malignant lesions. ATU with irregular uptake pattern (2/2) and in cases referred for cervical lymph node metastasis of unknown origin (3/5) were frequently associated with malignant lesion (p<0.05). Conclusion: ATU was not infrequently observed on 18F-FDG PET/CT, and the malignant risk of ATU was low. However, ATU with cervical lymph node metastasis or with irregular pattern on PET/CT would be further evaluated by the histopathologic examination.