Police are investigating a disturbance in Burgess Hill which left a man with a fractured skull.
At 11pm on Saturday, June 2, in Station Road, the man, who lives locally, was punched in the face, police said. He was also kicked and fell to the ground.
The disturbance, which police said involved a number of other people, happened at the junction with Church Road.
The man, who also suffered internal injuries and extensive facial bruising and abrasions, was taken to the Royal Sussex County Hospital in Brighton.
Another man, aged 55, who also lives locally, was treated for minor injuries, police said.
No arrests have been made at this stage but police enquiries are continuing, a spokesman said.
Detective Constable Michael Wyborn added: “A dispute arose between several other people at this location for reasons which are currently unclear, and the victim of this vicious attack sustained serious injuries when he apparently intervened.
“If you saw what happened or have any other information, please contact us online or by calling 101, quoting serial 1540 of 02/06.
“You can also contact the independent charity Crimestoppers anonymously on 0800 555 111.”