John Fell, 71, was reported missing from his home in Horsforth earlier today.
He was last seen at Shipley railway station at around 10.30am.
A West Yorkshire Police spokesman said: "John is described as extremely vulnerable and may seem confused or disorientated. He does carry a tracker device which is believed to have powered down since its last activation earlier today.
"Police and family are concerned for his welfare and officers are continuing enquiries to locate him."
Also in news: Man left critically ill after 'punch' in Leeds streetJohn is described as being of medium build and about 6ft 2in.
He is believed to be wearing a green jacket and dark trousers.
Anyone who can help is asked to contact police on 101, quoting log *0565 of 21 May.
Alternatively, use the other Contact Us options on the force website.