Primary central nervous system diffuse large B-cell lymphoma (PCNSL) is a rare B-cell non-Hodgkin lymphoma (NHL) of the CNS with highly variable clinical presentation.1,2 It can mimic other CNS conditions such as Herpes simplex virus (HSV) encephalitis which may delay the diagnoses and treatment leading to poorer outcomes. Therefore, a high index of suspicion for PCNSL must be kept in mind in patients with unusual CNS presentation or suboptimal response to therapy. We present a patient with MRI findings and HSV PCR consistent with HSV encephalitis who had no improvement on acyclovir and subsequently was diagnosed...

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