The novel was the basis of a highly successful Oscar-winning film adaptation released two years later, whose screenplay was also written and produced by Blatty, for which he won an Academy Award.

In September 2011, the novel was reprinted by HarperCollins to celebrate its 40th anniversary, with slight revisions made by Blatty as well as interior title artwork by Jeremy Caniglia.

An elderly Jesuit priest named Father Lankester Merrin is leading an archaeological dig in northern Iraq and is studying ancient relics.

As Chris finishes her work on the film, Regan begins to become inexplicably ill. After a gradual series of poltergeist-like disturbances in their rented house, for which Chris attempts to find rational explanations, Regan begins to rapidly undergo disturbing psychological and physical changes: she refuses to eat or sleep, becomes withdrawn, and increasingly aggressive and violent.

Detective Kinderman begins investigating Father Karras, as they exchange disturbing stories of black mass Catholic theology and 1960s film stars.

Karras heroically surrenders his own life in exchange for Regan's by jumping out of her bedroom window and dies, regaining his faith in God as his last rites are read.

Blatty had considered writing a novel based on the Doe exorcism since he had written about it as a college student, even as he became successful as a comic novelist and screenwriter in the early 1960s, but his agent talked him out of doing so when he raised the subject.

He liked the way director Roman Polanski had kept the audience unsure whether the title character's concerns for her unborn child were genuine or not, but felt the ending was "schlocky", reducing the Devil to a joke.

As he wrote, the story, which Blatty originally envisioned taking place largely in a courtroom, instead moved to the locations of the events recounted and become darker, with the possessed Regan masturbating with a crucifix and screaming obscene words at the priests trying to cast the demon out, all to make the evil more uniformly repulsive to readers.

[2] Aspects of the Father Merrin character were based on the British archaeologist Gerald Lankester Harding, who had excavated the caves where the Dead Sea Scrolls had been found and whom Blatty had met in Beirut.

"[9] Another correspondence between life and art: at the time Blatty knew MacLaine, she, like Chris MacNeil, had a married European couple as household staff.

In 1973, the novel was adapted by Blatty for the film of the same name and directed by William Friedkin with Ellen Burstyn, Max von Sydow, Lee J. Cobb, Jack MacGowran, Jason Miller and Linda Blair.

It was directed by John Doyle and starred Brooke Shields, David Wilson Barnes, Richard Chamberlain, Emily Yetter, Harry Groener, Roslyn Ruff, Manoel Felciano, Tom Nelis, and Stephen Bogardus.

The new production was directed by Sean Mathias, designed by Anna Fleischle and starred Jenny Seagrove as Chris, Peter Bowles as Merrin and Adam Garcia as Damien.