Fulminant demyelinating disease is a heading that covers acute disseminated encephalomyelitis and its variant acute hemorrhagic leukoencephalitis (Hurst disease), severe relapses of multiple sclerosis (MS), variants of MS (tumefactive MS, Marburg variant, Balo concentric sclerosis, myelinoclastic diffuse sclerosis), and neuromyelitis optica-spectrum disorders associated with aquaporin autoimmunity. These categories of inflammatory demyelinating disease often prompt hospital admission and many necessitate intensive care monitoring due to the aggressive nature of the illness and associated neurologic morbidity. In this review, we highlight the discriminating clinical, radiographic, and pathologic features of these disorders. Acute management is often accomplished with use of high-dose intravenous steroids and plasma exchange. Aggressive disease may respond to immunosuppression. Prognosis for recovery varies among the disorders but most patients improve. Factors influencing outcome are also discussed.