"The Exorcist" is based on a novel of the same name written by William Peter Blatty, published in 1971. While the story itself is a work of fiction, it was inspired by a real-life exorcism case.

The novel and subsequent film adaptation were loosely based on the exorcism of a boy known by the pseudonym "Roland Doe" or "Robbie Mannheim." In 1949, the boy, who was around 14 years old, allegedly experienced strange occurrences, including unexplained noises, objects moving on their own, and even physical attacks. His family, unable to explain or handle these events, sought the help of Catholic priests.

The Catholic Church assigned several priests to perform exorcisms on the boy, with the process taking several weeks. The details of what exactly happened during these exorcisms are disputed, but it's said that the boy spoke in strange languages, exhibited abnormal strength, and showed aversion to religious objects.

The case attracted considerable attention at the time and later served as the basis for Blatty's novel. Blatty's work fictionalized and dramatized many aspects of the case, expanding it into a broader narrative that explored themes of faith, doubt, and the battle between good and evil.

While the actual events of the Roland Doe case remain shrouded in mystery and skepticism, "The Exorcist" became a cultural phenomenon, both as a novel and as a film, known for its terrifying portrayal of demonic possession and its impact on popular culture.