The man, who is in his fifties, was found on Upper Castle Street, in West Bowling in the early hours of July 31.
Officers said that he suffered significant injuries to his face and was taken to Bradford Royal Infirmary for urgent medical treatment. He remains in hospital.
Detective Sergeant Eleanor Buchanan, of Bradford CID, today said: “I would like to appeal to the public to come forward with any information about this incident. Our enquiries are ongoing to establish the circumstances as to how he was injured.
“Anyone with any information is asked to come forward and speak to the police via 101 quoting log number *130 of 31 July or alternatively contact Crimestoppers anonymously on 0800 555 111.”