This paper summarizes the usefulness and limitation of positron emission tomography (PET) with (18)F-fluorodeoxyglucose ((18)F-FDG) in the diagnosis and treatment of primary central nervous system lymphoma (PCNSL). The (18)F-FDG uptake in typical PCNSL is about 2.5 times higher than that in the normal gray matter, and the tumor can usually be identified visually. The (18)F-FDG uptake pattern and value provide useful information for differentiating PCNSL from other enhancing malignant brain tumors especially glioblastoma (GB). The (18)F-FDG uptake in typical PCNSL is usually homogenous, and the uptake value is significantly higher than that in GB. However, (18)F-FDG PET often fails to show the presence of tumor in the brain as (18)F-FDG uptake is faint in atypical PCNSL such as disseminated or nonenhancing lesions. (18)F-FDG PET is also useful for evaluating the treatment response at a very early stage after the initial treatment. Pretreatment and posttreatment (18)F-FDG uptake values may have a prognostic value in patients with PCNSL. In conclusion, (18)F-FDG PET is very useful in the diagnosis of typical PCNSL and can differentiate PCNSL from other malignant brain tumors. However, the usefulness of (18)F-FDG PET is limited in the diagnosis of atypical PCNSL.