Although the summer box office sweepstakes have yet to be fully tallied, one low-budget, star-deficient, little-heralded film can already be certified as that rarest of Hollywood phenomena, a sleeper hit.

"My Big Fat Greek Wedding," a PG-rated comedy written by and starring Nia Vardalos -- who's that? -- has confounded the expectations of movie experts and the usual rhythm of Hollywood releases (big opening weekend, bigger drop-off, video) by moving up on the box office charts this past weekend to No. 8.

The film, which opened in April -- about the time that the long-forgotten "Scorpion King" was rolling out on thousands of screens across the country -- is still building a head of steam with audiences. So far the movie, made for a minuscule $3.5 million, has taken in $40 million and is expected to top off at about $60 million.

"It was in the top 10 once before, when 'Star Wars: Attack of the Clones' opened," said Paul Dergarabedian, head of Exhibitor Relations Co., which tracks the box office. "I've never seen anything quite like this. This is the most competitive summer ever, and it's holding its own against all the big films out there."

"Greek Wedding" stars Vardalos as an approximate version of herself: a Greek American in her thirties who marries a WASP (John Corbett of "Sex and the City") and comically tries to integrate him into her loud, loving clan.

Unlike in many hit comedies, the story is neither raunchy nor particularly conflictual. In the film, as apparently in Vardalos's life, her family roasts a lamb on a spit on the front lawn, where all the Greek statuary reposes. Her fiance, out of love for her, submits to a Greek Orthodox baptism-as-full-body-soak.

Vardalos wrote the script out of desperation, after moving to Hollywood and failing to find a career. Vardalos, a former cast member of Second City, the Chicago-based comedy troupe, was born and raised in Canada, in Winnipeg's small Greek community. She moved to Los Angeles after marrying Ian (she doesn't want to reveal his last name), also a former Second City comedian. Two other Second City contemporaries, Rachel Dratch and Tina Fey, moved on to "Saturday Night Live." Vardalos says of her husband: "He's a genteel New Yorker. . . . He doesn't talk very much."

When Vardalos was offered only bad roles as a Latina (which she felt ought to go to Hispanic actresses) and even her own agent failed to recognize her at a party, she decided to write a one-woman play about her family, which had served as material for years. The play ran for weeks, and when one evening Tom Hanks's wife, Rita Wilson -- who is of Greek descent -- was in the audience, that proved a turning point.

Other producers had wanted to make the play into a film. One said, "Here's a big check. We'll make it Italian and get Marisa Tomei to play the lead," recalls Vardalos. "I said no. I just thought it was blasphemous to turn my family into anything else."

But Hanks and his producing partner, Gary Goetzman, were willing to produce the film and allow Vardalos to star. The budget came from a small outfit called Gold Circle Films. For months no distributor was interested in picking up the film, until a small independent, IFC Films, agreed to distribute it.

Marketing strategist Paula Silver worked on building word of mouth for "Greek Wedding" with a low-budget, grass-roots campaign. Vardalos and Corbett went on the road, appearing on dozens of local radio and television shows when national programs like "Today" and "The Tonight Show" turned them down. (Last week Corbett was finally on with Jay Leno, but even now Vardalos is not enough of a celebrity to get an invitation.)

"Nobody would take us, nobody wanted Nia," says Silver. "But in each city we found a unique situation to do something." She screened the movie at a Hellenic museum in Chicago and ran a contest in Bride magazine. In New York Rita Wilson held a private screening; Silver showed the coming-attractions trailer at bridal shows.

The movie opened four months ago on a few dozen screens and attracted the target audience of women 18 to 40. As theaters have been added -- another 88 last week -- the audience has grown and broadened.

"We are still amazed at how wide the age group goes -- from 7- and 8-year-olds to 14-year-olds to a date movie. It's not rural, not urban, there's no region driving it," says Goetzman.

Silver believes the time was ripe for a feel-good story about ethnicity and compromise in a country where that reality affects many families. "It's about every cross-cultural family," she says. "Here's a man who wants to marry someone's daughter, has to compromise, and he does. It makes you proud of ethnicity."

Says Dergarabedian: "My mom went with her friends to see it. . . . Everyone can relate to it. Whether you're Greek or not hardly matters."

It's the kind of film that has other studio executives around Hollywood looking with interest at how to beat the odds in a movie release. And it has Vardalos busy reading scripts and fielding offers for work.

"It's crazy," she said a few weeks ago, even before the film became the hit that it s now. She has two offers to write movies for big stars, but she insists she wants to act.

"The main thing is, my anonymity is gone," she says with a laugh. "How'd that happen?"

Nia Vardalos wrote the script and starred with John Corbett.Nia Vardalos in the feel-good sleeper of the summer, "My Big Fat Greek Wedding."