This movie is a strange part of my life. When I was five and staying with relatives, this movie came on and I was supposed to be in bed sleeping, but I was on the staircase watching this through the posts of the bannister. A pretty obvious word of advice is- don't let anyone under the age of 16 see this, as I seriously had horrible nightmares and problems at night for a good ten years afterwords. It affected me so much that I became facsinated with the religion behind the Exorcist and studied Babylonian religion in college. The demon that possesed Regan really existed in the babylonian pantheon- it's called Pazuzu and was known as the god of the foul south wind that brought disease and pestlilence. There is even a statue of it at the British museum. The comments everyone else has made here say it all- this is by far the most disturbing and unsettling movie ever made and can have a deep psychological impact on a person as I can personally attest. The book is excellent as well and worth a read and re-read.