A man sustained a fractured skull after being punched in the head and knocked unconscious during an assault in Ilkeston.
Police say the 49-year-old victim was confronted by a group of teenagers on the Market Place.
One teen punched the victim once to the head, knocking him unconscious and causing a cut which required stitches.
The incident happened at around 6.40pm on Monday February 13.
A spokesperson for Derbyshire police said: "The youth was wearing dark clothing and a hooded top.
"Another group of teenagers, who were in the Market Place at the time, have come forward and spoken to us and are not thought to have been involved."
Witnesses or anyone with information should call DC Gavin Owen on 101, quoting reference 17000063951, or Crimestoppers on 0800 555 111.