By Nicole Sperling
Updated June 11, 2009 at 12:00 PM EDT

When director Todd Phillips found out that The Hangover had bounced Pixar’s Up from the No. 1 slot — and completely annihilated Will Ferrell’s Land

of the Lost — he was in Ireland for the film’s Dublin premiere. “They gave us a standing ovation after the screening, which I’ve never seen before,” Phillips tells EW. “I think everyone was drunk.”

Back in America, even sober audiences are cheering. The bawdy bachelor-party

flick — which was co-produced and co-financed by Legendary Pictures and cost

just $31 million to make — hauled in an unexpected $45 million in its opening

weekend. Only two other R-rated comedies have opened to bigger numbers: Sex

and the City ($57 million) and American Pie 2 ($45.1 million). But both of

those were well-known franchises with huge built-in fan bases. All Hangover had

was journeyman supporting actor Bradley Cooper, L.A.-based comic Zach Galifianakis, and The Office’s Ed Helms. Oh, and a tiger.

Because Warner Bros. had no stars to sell, it relentlessly flogged the movie itself. The studio screened the film more than 400 times and sent the cast around the country to college campuses and other hit-making locales. They also showcased the film’s irreverent humor (Mike Tyson’s rendition of Phil Collins “In the Air Tonight,” for example) in age-restricted online videos and plastered the actors’ day-after mugs on posters all over the country.

So what does The Hangover’s phenomenal performance mean? First of all, that the buzz is still building. The movie brought in another $7 million on Monday, and

is now on track to earn at least $200 million. Director Todd Phillips and his Old School writing partner Scot Armstrong began developing the sequel more than

two months ago, though they remain mum on any details. (“I don’t want to put it

out there,” Phillips tells EW. “We’ll get ­attacked.”) And — thanks to this and

the Judd Apatow films — Hollywood may be willing to take more chances on

comedies without marquee names attached. “It’s good to know that you don’t have

to have stars [to get ­audiences],” says WB marketing president Sue Kroll. “But

if you don’t, you better have a great movie.”

Hangover‘s next test will be this weekend’s Denzel Washington, John Travolta-starrer The Taking of Pelham 1 2 3. Check out our box office predictions for our prediction of how it will fare.