The prognosis of lateral medullary infarction is mostly benign, but unexpected sudden death can occur even when the initial symptoms are benign. An 81-year-old man visited the emergency room with whirling-type dizziness and right-veering tendency. Brain magnatic resonance imaging (MRI) revealed a right lateral medullary infarction accompanied by a right cerebellar infarction. He died suddenly at 1 day after admission with only slight lesion progression on MRI. This case suggests that patients with a lateral medullary infarction should be carefully monitored during the acute period.