IT WASN'T so much that the American Film Institute Theater was an afterthought of the I Kennedy Center, for it already had been operating in other places around town for three years when its official house opened in April 1973.

The main problem was that money was running low, too low for turning the cinder-block backstage area into a conventional movie theater. The inspired decision was to make the most of the backstage look, even to the point of hanging automobile hoods on the walls to baffle the sound.

At first the audience seemed as baffled as the sound. (Some still feel the Center treats film like a shabby cousin, dating from its attempt to stop the screening of "State of Siege" at the AFI opening gala because it might offend the Kennedy family, being about an assassination. Boycott threats by several leading directors ended the affair.) But program director Michael Webb, a veteran of the British Film Institute, gradually built up attendance with a stunning series of films from all over the world, ranging from pioneer Swedish pictures to the newest works of top international directors.

Today the AFI membership stands at 111,678, and Webb's original pocket-size program guide, modeled on BFI's witty guide, has become a bimonthly magazine called Preview. Theater director Michael Clark has lined up for September the second half of his giant series on women filmmakers, the Washington premiere of "The Uprising," a docudrama about Nicaragua; a treasury of black comedy and another round of misappreciated American films, from "Lilith" to "The Chase."

Under the first AFI director, George Stevens Jr., thousands of old films have been restored, grants have gone to many young filmmakers, including Terence Malick for "Badlands," a film study center was established in Los Angeles, and the booming national magazine American Film was launched. Stevens was succeeded in 1980 by Jean Firstenberg, who has been making AFI even more transcontinental than it has been.

In any case, from its inception in 1967, AFI has created a new serious film audience in Washington, an audience that no longer need depend on the vagaries of repertory film at second-run houses. AFI programming has been criticized, both for favoring the too-familiar and for reviving films best left forgotten. That's show biz.