Multiple sclerosis (MS) is an inflammatory demyelinating disease of the central nervous system. We report the case of a 50-year-old man who presented with progressive gait ataxia. Brain magnetic resonance imaging (MRI) on fluid-attenuated inversion recovery revealed a hyperintense lesion in the right temporal white matter. The spinal cord showed a long hyperintense lesion between the vertebral levels C6 and L1 on T2-weighted MRI. Biopsied tissues from the brain lesion demonstrated features of inflammatory demyelination with preservation of astrocytes, consistent with typical MS. This is the first reported case of pathologically proven MS with longitudinally extensive spinal cord lesions.