Acute disseminated encephalomyelitis (ADEM) is a disease of the central nervous system (CNS) mainly affecting children. It usually occurs within 2 days to 4 weeks following a triggering factor such a viral infection or an immunization. Clinical presentation is characterized by an acute encephalopathy and by multifocal neurologic abnormalities. In the absence of specific biologic marker, the diagnosis of ADEM is based on clinical, biological and radiological data including cerebrospinal fluid (CSF) analysis and magnetic resonance imaging (MRI). Brain MRI typically shows multifocal lesions predominantly involving the white matter. Treatment is based on high doses of steroids. Intravenous immunoglobulins or plasmapheresis are sometimes required. The prognosis is usually favorable but neurological sequellae can occur.