(18)F-flourodeoxy-glucose (FDG) PET/computed tomography (CT) is most useful in the evaluation of primary central nervous system (CNS) lymphoma, important in diagnosis, pretherapy prognosis, and therapy response evaluation. Utility in working up gliomas is less effective, and FDG PET/CT is most helpful when MR imaging is unclear. FDG avidity correlates with the grade of gliomas. FDG PET/CT can be used to noninvasively identify malignant transformation. Establishing this change in the disease process has significant effects on patient management and survival outcome.