18Fluoro-deoxyglucose positron emission tomography (FDG-PET) imaging has been widely incorporated into the management of systemic non-Hodgkin lymphoma. However, its utility in primary central nervous system lymphoma (PCNSL) is unclear. We report 28 PCNSL patients who underwent brain FDG-PET to inform clinical management. Baseline PET imaging demonstrated hypermetabolism consistent with aggressive lymphoma in 75% (9/12) of patients. FDG-PET used for response assessment or the interpretation of nonspecific magnetic resonance imaging abnormalities showed hypermetabolism in 50% (10/20) of patients, which was important in guiding further management. This data suggests that FDG-PET imaging in PCNSL is useful and may have a role in response assessment.