A teenager was stabbed outside the KFC fast food restaurant in London Road, Brighton, last night (Saturday 22 July).
The 18-year-old was treated locally before being flown to hospital in London, Sussex Police said.
He is believed to have been stabbed during a fight outside the KFC, with the police being called shortly before 8.30pm.
Officers were still investigating late last night.
Anyone who saw what happened is asked to phone Sussex Police on 101 or email firstname.lastname@example.org.
To report the details online, click here. The crime serial number is 1363 of 22/07.
Alternatively witnesses can call the independent charity Crimestoppers anonymously on 0800 555111 or visit www.crimestoppers-uk.org.