A 34-year-old woman was admitted to hospital after sustaining a head injury in a motor vehicle accident (day 1). No signs of neurological deficit, skull fracture, brain contusion, or intracranial bleeding were evident. She was discharged without symptoms on day 4. However, headache and nausea worsened on day 8, at which time serum sodium level was noted to be 121 mEq/L. Treatment with sodium chloride was initiated, but serum sodium decreased to 116 mEq/L on day 9. Body weight decreased in proportion to the decrease in serum sodium. Cerebral salt-wasting syndrome was diagnosed. This case represents the first illustration of severe hyponatremia related to cerebral salt-wasting syndrome caused by a minor head injury.