Patients with primary central nervous system lymphomas (PCNSLs) present with nonspecific clinical symptoms, which makes correct imaging evaluation essential for diagnostic and therapeutic management. In this work, we examined an 81-year-old man with recently discovered PCNSL using F-FDG PET/MRI, and we were able to differentiate between 2 lesions-PCNSL lymphoma extension and a recent ischemia. Our work shows that ischemia should be considered as a differential diagnosis for lymphoma progression. Although F-FDG PET or MRI alone cannot always give unambiguous solution, PET/MRI can greatly improve the diagnosis accuracy and help decide on the appropriate patient management.