LOS ANGELES — Warning: Smoking may be dangerous to your movie rating.

In a significant change to its movie ratings system, the Motion Picture Association of America on Thursday said portrayals of smoking would be considered alongside sex and violence in assessing the suitability of movies for young viewers. Films that appear to glamorize smoking will risk a more restrictive rating, and descriptions of tobacco use will be added to the increasingly detailed advisories that accompany each rated film.

Anti-smoking groups, already successful in much of the country in banning smoking in bars, restaurants and other public places, have ratcheted up the pressure on Hollywood in recent years to purge movies of images that might promote tobacco use. Some have even demanded that virtually any film with smoking be rated R, shutting out those under 17 unless they are accompanied by a parent or adult guardian.

Under the new policy, a film's rating will consider all tobacco use, rather than just teenage smoking, as in the past. But the board stopped short of guaranteeing that tobacco use would be considered as heavily as sex, violence or drug use in assigning a rating. (Film ratings are assigned by a panel of about a dozen parents through an apparatus called the Classification and Ratings Administration, and overseen by both the Motion Picture Association and the National Association of Theater Owners.)

"It's an art, not a science," said Joan Graves, chairman of the ratings board, of the actual weight that would be placed on smoking in assigning a rating. "It all depends on how impactful the smoking is."

In deciding whether a tougher rating is appropriate, board members will be expected to consider whether smoking is pervasive, whether it tends to be glamorized, and whether the context or historical fact mitigates the portrayal. George Clooney's portrayal of a chain-smoking Edward R. Murrow in "Good Night, and Good Luck," said Graves, would likely still get the PG rating it received in 2005, because it was historically accurate. The rating, however, would now be likely to include an advisory about pervasive smoking.