Tomasz Tarkowski, 34, was reported missing from Leeds General Infirmary last night.
He left the building at about 10.43pm through the main entrance and turned right at the mini-roundabout.
Mr Tarkowski, who is Polish, is described as white, 5ft 7ins tall, medium build, with a bald head. He was wearing a zip-up jacket of shiny material, dark blue jeans, and a distinctive trainers which are bright blue with green soles. He was carrying a royal blue rucksack and a black rucksack. He speaks limited English.
He had a 1cm cut to his left eyebrow covered with a plaster.
Detective Inspector Susan Maclachlan, of Leeds District CID, said: "Tomasz left the hospital before his head injury had been fully assessed and we are concerned for his welfare. He often frequents the city centre and we have officers out looking for him.
"We would like to hear from anyone who has seen him or who knows his current whereabouts so we can check that he is okay."
Anyone with information is asked to contact Leeds District CID via 101 quoting log number 2054 of June 25.