Police discovered an injured man after being called to reports of screaming coming from woodland in Garston at the weekend.
Hertfordshire Constabulary attended Alban Wood the near Woodgate at 9.17pm on Friday after receiving calls from alarmed residents who heard a disturbance from the area.
Officers soon found an 18-year-old with head wounds and launched a search of the area including door-to-door enquiries.
The teenager was later taken to Watford General Hospital for treatment.
The police investigation was ended after the young man told officers he had fallen and no evidence was found of violence being involved in the incident.