It was a film that terrified millions of cinema-goers and one scene in particular chilled them to the bone.
Demonically possessed Regan is wheeled into a hospital operating theatre to have medical tests carried out.
A catheter is inserted into her neck through a major artery to determine what is causing her strange behaviour.
As the tube is being inserted and little Regan grimaces, blood spurts out of her neck.
Despite it being a medical procedure and nothing to do with the occult it has regularly been viewed as one of them most chilling in the film.
In reality, the scene also hides a chilling secret – a real life serial killer.
Paul Bateson was a radiographer and had come to the attention of the classic horror film’s director, William Friedkin, when he visited a hospital to carry out some research.
After watching Bateson perform a cerebral angiography, where a catheter is inserted in a major artery to take images of the brain, he was inspired to include a similar scene in The Exorcist.
He invited Bateson to work as an extra on the film and to act as the technician while the procedure was carried out.
Bateson is the man in the background, calmly explaining to Regan what will happen to her as the tests are carried out.
He has most of the dialogue in the unnerving scene and seemed to have an incredibly calm and kind bedside manner.
This is something Bateson’s colleagues at New York University Medical Center had regularly commented on.
Just six years after his role in the iconic horror film, Bateson himself became the stuff of nightmares when he murdered for the first time.
In 1979 he was convicted of killing film industry journalist, Addison Verrill, and sentenced to 20 years in prison.
But before his murder conviction, police were aware of Bateson and believed he was involved in the murders of several gay men in Manhattan, New York.
Verrill, who wrote for Variety, was found dead in his flat in New York. He had been beaten and stabbed to death.
There was no sign someone had forced their way into his home and there were empty beer cans.
Following his death, one of his friends, journalist and gay activist Arthur Bell, wrote a piece in The Village Voice urging the killer to come forward to police.
Instead, eight days later, Bell himself was contacted by a man claiming to be the killer.
The caller claimed he had been sober for three months and was “gay, needed money and was an alcoholic”.
He had gone to a bar and met Verrill, who offered to buy him a drink.
The two had several drinks and some drugs and ended up at a club where they continued to party.
The caller said: “I didn’t realise he was such a superstar and I wanted to go home with him.”
The two ended up back at Verrill’s apartment, drank some more, took cocaine and had sex – but then Verrill made it clear he didn’t want the relationship to go any further.
“I needed money and I hated the rejection. I decided to do something I’d never done before”, he told Bell.
The caller claimed he had hit Verrill with a frying pan and when he was knocked out, stabbed him.
He stole some cash and his credit card, passport and some clothes before fleeing the apartment.
He used the cash to buy booze and drank all the following day and told Bell that he wanted to “atone” for several other crimes.
Bell contacted police, who waited with him for the caller to ring back.
There was a second phone call but this time from a man calling himself Mitch – and he identified Bateson as the killer.
Police arrested Bateson, drunk, at his apartment and he provided officers with a written confession of the murder.
Officers had also been investigating a series of murders where gay men had been killed in New York.
Bateson always publicly denied having anything to do with these crimes but prosector William Hoyt claims he had confessed his involvement to a friend, Richard Ryan.
The judge at his trial sentenced him to 20 years for Verrill’s murder but said the connections to the other killings were “too ephemeral”.
The Exorcist director Freidkin knew nothing of the murder conviction until he read an article about it and visited Bateson in Rikers Prison.
Bateson told the director, who remembered him as “nice young man” that he was being offered a plea deal.
He said he had been told if he confessed to the murders of the other gay men his sentence would be shorter.
But he told Freidkin he wasn’t sure if he would accept it.
Bateson served 24 years and three months before he was granted parole in 2003 – the day after his 67th birthday.
Nothing is known of Bateson since his release from prison but a social security death index seems to show he passed away on September, 2012.