The annual Costume Institute gala is never a casual affair, of course, but add the extra inducement of dressing in the spirit of Paul Poiret—a.k.a. the self-proclaimed "King of Fashion"—and you can count on the "Party of the Year" going into sartorial overdrive. Stars from every galaxy came out in variations on the early twentieth-century couturier's tiered dresses, feathered headbands, and cocoon jackets—even the occasional cape. "It's a fashion celebration, that's for sure," said Shalom Harlow, in Viktor & Rolf. "It's a decadent industry and this a decadent night." Indeed. At the top of the museum's imposing stairs sat four peacocks in a giant gilded birdcage surrounded by a hedge of lush red roses. Taking it all in, a wide-eyed America Ferrera said, "I'm as overwhelmed as Betty would be."

The evening's hosts, Vogue's Anna Wintour, Balenciaga's Nicolas Ghesquière, and Cate Blanchett, greeted guests at the end of the red carpet. "It's not my first time," said Mick Jagger after saying his hellos. "I've been before and it's always a fantastic event." A Narciso Rodriguez-clad Julia Louis-Dreyfus, on the other hand, confided, "I have to admit, I'm just learning." Examining the Poiret dresses arrayed vignette-style in glass vitrines, she exclaimed, "Look at that, how modern! I would buy that now."

During cocktail hour in the Carroll and Milton Petrie European Sculpture Court, Kirsten Dunst, in vintage YSL, smooched her new paramour, Johnny Borrell of Razorlight; Julianne Moore and Stefano Pilati exchanged hugs with Salma Hayek and François-Henri Pinault; and Tom Ford mussed Jack McCollough's new mod 'do. Watching from the sidelines, actress and rock 'n' roller Juliette Lewis admitted to being a tad shy. "I'm a bit of an outsider at these things," she said, "especially because I've been living on a bus for the past six months with my band, The Licks. But Tommy Hilfiger asked me to come, and it's nice to dress up once in a while."

Four trumpeters gave the universal signal for "make your way to dinner," and soon the fashion flock was on the move again. Inside the Engelhard Court, the walls and oversize lanterns were decorated in lavish Poiret style. "This is one of the best years for the gala," said Vogue's André Leon Talley, in a signature cape, with Renée Zellweger at his side. "It's so relaxed. Everyone is at ease." While waiters rushed to pass out the Poiret rose-shaped salmon appetizers, the actress mused about Talley, her personal king of fashion for the evening: "He's holding court; I'm just a loyal subject. All I saw was miles and miles of fabric and then him. Of course it was him."

Click here for more exclusive pictures from cocktail hour at the Costume Institute "Party of the Year"

Click here to see our feature on Paul Poiret and his extraordinary designs

— Sarah Cristobal