It said the new label does not immediately carry the commercial stigma of X, which is so strong that many theater chains - including the nation's two largest - won't show X-rated films and many newspapers won't carry ads for them.
The MPAA also said it will give film companies and theater owners more details on films it rates R, noting violence, sex and language. The PG-13, PG and G ratings will remain the same.
Universal Pictures' X-rated "Henry & June," a historical drama featuring some sex scenes, will be the first film bearing the NC-17 rating, Universal said.
"It's a great step," said Philip Kaufman, director of "Henry & June." "I'm just really delighted that we can show this film uncut in America and show the film the way I made it. It is not intended as a prurient film."
Critics of the MPAA had been trying for months to change the system. They said the X rating, never copyrighted, had become synonymous with pornography because makers of hard-core adult films used it freely.
The new rating will be copyrighted. That means it cannot be used by porno producers unless they submit their films for MPAA review, something they never do because it costs up to $2,000 per film.
Film critics Gene Siskel and Roger Ebert, hosts of a syndicated movie-preview show, applauded the new category.
"It represents a victory for everyone who cares about truly adult films. I'm using adult here in the sense of mature, thoughtful, serious and intelligent - not in the sense of `dirty movies,' " Ebert said.
"It's good news," said Dan Harkins, president of the 23-theater Harkins Theatres chain in Phoenix. "It takes some of the politics out of the rating system and our advertising will be strengthened."
It was unclear whether newspapers and TV and radio stations would be more inclined to carry ads for movies bearing the NC-17 mark.
Laura Morgan, a spokeswoman for the Los Angeles Times, predicted advertisements would be judged on a combination of their artwork and critics' opinions on the movies' artistic worth.
William Kartozian, president of the National Organization of Theater Owners, said the initial response to the new rating had been positive, and he expected most theaters to show NC-17 films. He also said some states may amend the NC-17 rating to NC-18, barring children under 18.
In 1984, the PG-13 rating was added between PG and R, prompted by "Indiana Jones and the Temple of Doom."
MPAA President Jack Valenti said the rating system had been justifiably criticized. "It's no one's fault, but the X rating has a patina, a complexion which frankly we never intended it to have," he said.