Ian Brady could be showing early signs of dementia, his mental health advocate has said.
Jackie Powell said the behaviour of the Moors murderer had become increasingly bizarre and medical experts believed he was now seriously ill.
The revelation has prompted fears that he may become unable to remember where he buried Keith Bennett, the only one of his five young victims whose remains have never been found.
Brady, 76, was jailed for life in 1966 along with his girlfriend Myra Hindley for abducting, abusing, torturing and killing five youngsters in Manchester before burying them on nearby Saddleworth Moor.
He has been held at Ashworth high-security psychiatric hospital in Liverpool since 1985.
Ms Powell, who was visited Brady regularly since 1999, told the Daily Mirror that during a recent meeting with him she saw signs of dementia, and believed he was nearing the end of his life.
But she said that Brady wouldn't accept that there was anything wrong and refused to undergo tests that could confirm whether he has Alzheimer's disease.
She told the newspaper: "Brady has to be in control and getting dementia is his worst nightmare.
"No one who meets him could ever deny that he had a very sharp mind. He is highly intelligent and insists on dealing with things on his own terms. He never wants to expose his feelings so he is terrified of losing his self-control.
"Everything he does has always been very calculated and he could not imagine anything worse than his own mind slipping away from him. He'd never admit it, but I believe he is frightened about what is happening to him."
Ms Powell said that Brady had withdrawn into himself, and that he now seems to be "waiting to die".
She also said she believed it highly unlikely he would ever reveal where Keith Bennett is buried, saying that he refuses to even discuss it.
Last night a solicitor for the Bennett family, who have campaigned for years for Brady to reveal where he buried the 12-year-old, appealed for him to give up his secret while he can.
John Ainley told the Mirror: "If he deteriorates and is not able to impart this information then it may be that Keith is never found."
Brady was denied a transfer from Ashworth to a normal prison last summer after a tribunal declared him to still be insane.