Acute transverse myelitis had many names and definitions, based primarily on clinical criteria. The role of MRI in the exploration of myelitis has increased recently after the individualization of neuromyelitis optica (NMO) in 2004. This approach has enabled clarification of the diagnostic and prognostic value of acute longitudinally extensive transverse myelitis (LETM), defined by an extensive T2 lesion affecting three vertebral segments in the sagittal plane. The limitations of this definition, the multiplicity of terms used to characterize it as well as the large number of etiologies associated with it led our group of experts to clarify its etiology and nosology. We conducted a national survey on this subject in order to propose a new definition of LETM. Additional first- and second-intention examinations were determined according to the clinical context. Infectious/para-infectious, inflammatory or paraneoplastic causes can thus be identified. To determine within a short time the cause of LETM is essential, since most of its causes are severe and require urgent treatment.