A 54-year-old man presented with primary position upbeat nystagmus and wall-eyed bilateral internuclear ophthalmoplegia. He also showed bilateral limb ataxia and impaired horizontal gaze. Upbeat nystagmus obeyed Alexander's law and attenuated by visual fixation and disappeared by convergence. Brain magnetic resonance imaging showed acute infarction in the bilateral paramedian midbrain involving the crossing of brachium conjunctivum. Multiple mechanisms including the interruption of central vestibulo-ocular projections from anterior canal may be postulated in upbeat nystagmus of this patient.