A 76-year-old man with essential hypertension abruptly presented with slight left-sided leg weakness, despite normal strength in the other extremities. Left-sided Babinski's reflex was detected. There were no other neurologic abnormalities. Cranial magnetic resonance imaging demonstrated a small infarction in the lower lateral medulla oblongata on the left side. Cranial magnetic resonance angiography demonstrated an absence of flow of the left vertebral artery. He became asymptomatic within 10 days under intravenous antiplatelet agent. The corticospinal tract fibers innervating the lower extremity caudal to the pyramidal decussation might be involved. We emphasize that this is a first reported case of pure motor monoparesis in the leg due to lateral medullary infarction.