Police are looking for a 33-year-old man who walked out of hospital in Cottingham, near Hull.
Wilson Mallon was being treated at Castle Hill hospital but left this lunchtime.
Mr Mallon, who walks with a limp, has not had his medication and police are warning that he may be agitated and acting "erratically."
People are advised not to approach him and call police first.
He has an Irish accent, is white, around 5ft 6ins, slim, with short dark hair and stubble on his face.
He was last seen wearing a blue checked shirt, trainers and turned-up blue jeans.