A 43-year-old woman was taken to the accident and emergency department after a high-speed motor vehicle collision. She had only minor external injuries, but developed an episode of hypotension, which rapidly responded to fluid infusion. Chest X-ray showed a widened superior mediastinum. Computed tomography confirmed thoracic aortic injury. In patients with high-speed deceleration injury, even transient hypotension should be taken seriously and a diligent search for the source of bleeding should be immediately undertaken.