Background: Intravascular large B-cell lymphoma (IVLBCL) is infrequent and aggressive, clinically depicted by an almost exclusive growth of large cells within the lumen of blood vessels of all sizes. The clinical manifestations are diverse, encompassing many non-specific signs and symptoms such as fever of unknown origin, neurological symptoms, and skin lesions. Case Presentation: We report a case of a 49-year-old lady with unusual IVLBCL of the central nervous system. She presented with rapidly progressive dementia preceded by seizures and a short history of altered sensorium. There were no cutaneous lesions. Plain computed tomography brain showed no significant abnormalities. Cerebrospinal fluid analysis was normal except for mildly raised protein. Antinuclear antibody was positive 1:320 but the remaining autoimmune workups were negative. Electroencephalogram showed cortical dysfunction with occasional sharp wave at the right frontoparietal region. Other dementia workups were unremarkable. Magnetic resonance imaging brain revealed non-enhanced biparietal gyri hyperintensities which may represent encephalitis changes. She was empirically treated for viral encephalitis, however, there were still recurrent seizures despite adequate anti-seizure medications with minimal improvement of symptoms. Subsequent admissions noticed bicytopenia with elevated lactate dehydrogenase. Bone marrow aspirations and trephine biopsy disclosed High-Grade Mature B Cell Lymphoma germinal center B-cell Type. She was subsequently managed by the hematology team and started with conventional rituximab, cyclophosphamide, doxorubicin, vincristine, and prednisolone therapy. Fortunately, she is currently on the road to recovery. Conclusion: High index of suspicion is warranted to diagnose early in order to have a better prognosis.

Keywords: IVLBCL, rapidly progressive dementia, seizures, altered sensorium, bicytopenia.