Traumatic posterior circulation aneurysms are rare, especially in children. They are typically associated with severe trauma and are thought to result from either direct osseous injury or stretching or compression of an artery against the tentorium. They may grow, rupture, or both. Although spontaneous thrombosis may occur, it rarely results in complete occlusion. We report the spontaneous and complete thrombosis of a large posterior cerebral artery aneurysm in a child who presented after minor head trauma. Five-year follow-up documents the complete occlusion of the aneurysm.