Police believe a man who was struck by a car while lying in a Derbyshire road had been attacked beforehand.
The accident happened at about 0225 GMT on Sunday on Derby Road in Borrowash.
The people in the car stopped, administered first aid to the injured 20-year-old and called emergency services.
He is undergoing surgery in hospital in Sheffield. An 18-year-old man has been arrested on suspicion of causing grievous bodily harm.
He remains in custody and is being questioned by detectives.