Hippocampal infarction is relatively rare. Many different diseases can mimic hippocampal infarction including transient global amnesia, Alzheimer's disease, epilepsy, encephalitis, and encephalopathies. An 89-year-old man was transported to our hospital for altered consciousness. Diffusion-weighted magnetic resonance imaging revealed slightly intense signals in the hippocampus with a mildly decreased apparent diffusion coefficient. Serial magnetic resonance imaging revealed features of hippocampal infarction. Symptoms and cognitive function gradually improved with rehabilitation, and he was transferred to a rehabilitation facility on Hospital Day 38. Hippocampal infarction is rare in patients with altered mental status, but should be considered when magnetic resonance imaging shows findings suggestive of this condition. Other differential diseases should be ruled out by serial magnetic resonance imaging and observation of the clinical course.