A man was knocked unconscious in a pub car park in Crawley suffering a bruise to the brain.
The 21-year-old man was confronted by a group of about five men outside The Windmill in Gossops Green at about 9.30pm on Saturday (May 31).
He was punched in the face and fell to the ground before being taken to hospital for treatment.
The victim, from Crawley, also suffered cuts to his lip and head.
PC Adam Tidy said: "There were a lot of people in and around the car park at the time of the assault so someone must have seen what happened.
"We need to speak to anyone who was in the area to find out what they know. Please get in touch with us with any information you have."
Anyone with any information should call 101 quoting serial 1569 of 31/5, email firstname.lastname@example.org or call the independent charity Crimestoppers anonymously on 0800555111.