Police are looking for Alan Ruddy, 29, from Carlisle, who left the hospital at around 12.20am this morning (July 21).
He was last seen walking towards Lancaster city centre and police said his disappearance is "very worrying".
Alan is white, 5ft 8in tall, of stocky build with a pale complexion and a shaved head. He was last seen wearing a dark-grey tracksuit, a large black jumper and carrying a rucksack and a carrier bag.
He has links to the Lancaster, Morecambe, Carlisle and Grange-over-Sands areas.
Police are appealing to anyone with information about his whereabouts to come forward.
PC Zak Sly, of Lancaster Police, said: "We are appealing for information after a man went missing from Lancaster today (Tuesday, July 21).
"Alan’s disappearance is very worrying and we would ask anyone who has seen him or knows where he is to come forward.
"We believe he might have travelled to the Cumbria area. If you have seen him please get in touch immediately.”
Anyone with information can call 101 or (01524) 596627 quoting log 0102 of July 21. In an emergency call 999.