Background MS is a demyelinating CNS disease and has distinct clinical and radiological features. Neuromyelitis optica spectrum disorder (NMOSD) is an antibody related auto-immune disease known for invalidating episodes of myelitis and optic neuritis. Objective Reporting the case of a 29-year old woman with a disease course typical for relapsing remitting MS with consistent radiological and spinal fluid findings, who developed longitudinally extensive transverse myelitis (LETM) with positive aquaporin 4 antibodies, fulfilling the diagnostic criteria for NMOSD. Methods Case report. Results LETM is not consistent with MS. Consider NMOSD even in patients with typical MS and check for aquaporin-4 antibodies, with important treatment consequences.