Intravascular large B cell lymphoma (IVLBCL) is a rare condition with a predilection for central nervous system involvement and is often misdiagnosed. This case report describes a 58-year-old gentleman who presented with paraparesis and subsequent dramatic neurological deterioration initially attributed to acute disseminated encephalomyelitis. Upon post-mortem examination, the correct diagnosis of intravascular large B cell lymphoma was reached. This condition has a poor prognosis but can be chemotherapy responsive. It is hoped that this case report will raise awareness of a rare and diagnostically challenging illness.