Wow, it was based on a true story.

I was watching a strange cable movie tonight called Possessed. It seemed so similiar to the Exorcist that I loooked it up on IMDB and discovered it was based on a real 1949 exorcism that occured in Maryland. This site goes into quite a bit of detail on it. Enjoy. Part I - The Haunted Boy: the Inspiration for the Exorcist