BACKGROUND: Determining the frequency of longitudinally-extensive transverse myelitis (LETM: T2-lesion ≥3 vertebral segments) in multiple sclerosis (MS) is essential to assess its utility in differentiating from aquaporin-4-IgG (AQP4-IgG) positive neuromyelitis optica spectrum disorder (NMOSD) and myelin-oligodendrocyte-glycoprotein-IgG (MOG-IgG) myelitis. We sought to determine the frequency of LETM in MS during a myelitis attack. METHODS: We identified Olmsted County (MN, USA) residents on 12/31/2011 with inflammatory demyelinating disease. Inclusion criteria were: 1) Clinical myelitis episode accompanied by a new spinal magnetic resonance imaging (MRI) lesion (≤6 weeks from onset); 2) MS diagnosis by 2010 McDonald criteria; 3) Seronegative for AQP4-IgG and MOG-IgG. MRI characteristics were determined. RESULTS: Sixty-seven patients (median age at myelitis: 41 years [range, 16-65]; 76% females) with 92 myelitis attacks accompanied by a new MRI spinal cord lesion were identified. The frequency of LETM was 0%. The median T2-hyperintense lesion length in vertebral segments was 1.0 (range, 0.5-2.5) and 82/92 (89%) were peripheral in location on axial sequences; 58% had associated gadolinium enhancement. Two patients (2% of attacks) had multiple short lesions resembling LETM on sagittal images but axial sequences confirmed multiple non-contiguous short lesions. CONCLUSION: LETM is rare in adult MS myelitis and its presence should prompt evaluation for AQP4-IgG, MOG-IgG or other etiologies. Careful scrutiny of axial images is important as coalescence of multiple short lesions may lead to the artifactual appearance of an LETM.