Was The Exorcist based on a true story?

It was based on a story that may or may not be true.

The movie The Exorcist was based on a novel of the same name by William Peter Blatty. The novel is based on the real-life attempt of a priest to exorcise a demon from a young boy (the boy was around the same age Reagan was in the movie). Whether or not the real-life case was a true occurrence of demonic possession, or an elaborate hoax, or just a matter of misdiagnosed mental illness, is debatable.

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