Behind the Green Door (1972)
Behind the Green Door is a 1972 American feature-length pornographic film, widely considered one of the genre's "classic" pictures and one of the films that ushered in The Golden Age of Porn. Featuring Marilyn Chambers, who became a mainstream celebrity, it was one of the first hardcore films widely released in the United States and the first feature-length film directed by the Mitchell brothers. It was adapted from an anonymous short story of the same title, which was circulated by means of numerous carbon copies. The story's title makes reference to the 1956 hit song "Green Door".
More at Wikipedia.