And so she did what an increasing number of New York women are doing every night of the week: She wiggled into a pair of Spanx, the nylon and spandex undergarments that cinch a gal’s waist and thighs, eliminating underwear lines and shaving off anywhere from 2 to 10 pounds, depending on whom you ask. “They look like The Crying Game, but I do feel better when I’m wearing them,” Kate said. And she knows she is not alone. “If anyone ever catches a glimpse in the bathroom, they’re like ‘Omigod, I have those in black and I love them!’”

“They’re like hosiery crack!” said Suze Yalof Schwartz, an editor at Glamour who puts Spanx on all the women she styles for the magazine’s TV makeover segments. “They’re addictive. I would say hold off as long as you can.” She estimated that she herself owns 10 pairs of the tights. “Who knows, I’m scared to even go into that drawer!”

>> Spencer Morgan: I Put My Chestnuts In Spandex Storage—It Felt … Creepy!

Scores in Manhattan society now swear by Spanx, the moderately priced innovation of one Sara Blakely, 36, an entrepreneur based in Atlanta who has been widely credited with reintroducing girdling to the masses, thanks to comfortable fabrics; cheeky packaging; 100 different styles that reach as far up as the bust and as low as the ankle, eliminating the flabby overflow known as “muffin top”; and plenty of celebrity endorsements. Oprah was an early convert (“Spanx really changed the way I wore clothes. … I’ve given up panties,” she once told her audience) and Gwyneth Paltrow owned up to wearing two pairs at a time after giving birth. The company, which launched in 2000, recorded over $150 million in sales in 2006.

Oddly for a city that long scorned nylons, Spanx are particularly coveted in New York, where they have become ubiquitous at benefits and photo shoots. “It’s such a fashionable, stylist-oriented city,” said Ms. Blakely, adding that she does the most business here. “And the stylists have become Spanx’s No. 1 fan.”

Daniel Lawson, costume designer for NBC’s forthcoming series Lipstick Jungle, has dressed the show’s entire cast—some of who request it specifically—in the special undergarment, as well as Kate Winslet in a forthcoming movie. “We just Spanx everybody right up!” he said. “Without a lot of effort, they take off five, eight pounds immediately.” Not to mention “everything slips on smoothly. It’s become the Kleenex of the girdle world.”

“I’m in my 40’s, and I’d say definitely probably 80 to 90 percent of my friends wear them,” said the designer Pamella Roland. “Especially with evening gowns.”

‘Not Your Grandmother's Girdle’

Ms. Blakely is not surprised that New Yorkers, surely among the skinniest women in America, are her biggest fans. She was a size two when she invented Spanx, she said, and she was primarily trying to eliminate the dread Visible Panty Line.

“I didn’t like the way my own butt looked in white pants,” she told The Observer in a phone interview. “I went shopping for body-shapers at the age of 27, and I was completely horrified by what was out there.”

She started her company with $5,000 she’d saved selling fax machines door-to-door. The response was immediate. “There was a whole new interest from the consumer in wearing shapewear that wasn’t your grandmother’s girdle,” said Elizabeth Hospodar, divisional merchandise manager for Intimate Apparel at Bloomingdale’s. “Imitations and new innovations have proliferated, but Spanx remain the best seller,” said Ms. Hospodar, who estimated that nationwide, Bloomingdale’s Spanx sales have increased between 30 percent and 50 percent in the past two years. Ms. Blakely said she gives Spanx, which retail for $20 to $40, to friends and acquaintances like socialite songwriter Denise Rich and socialite designer Tory Burch. Over the summer, she said, “Madonna’s stylist called us and said, ‘Madonna’s such a fan of Spanx, she wants her entire outfit for Live Earth to be made out of Spanx material; can you FedEx it to us?” And so they did.

Meanwhile, a Spanx representative estimated that sales have more than tripled in the past two years in New York alone.

Could the confinement of Spanx actually be a liberation of sorts, allowing us to cheat gravity for an evening, skip the gym occasionally or at least eat a big lunch?

“In New York it’s all about perfection,” said a prominent socialite who asked not to be named. “But the fact is, we’re not in perfect, toned shape.” With Spanx, she said, “you feel like you’re being covered before you put on a dress. You can dance. You don’t feel so self-conscious about your stomach sticking out.”

Whether they’re nude “High-falutin’ Footless,” which extend to the bra line and hook to the bra, or black opaque “High-waisted Tight-End Tights,” Spanx are relatively easy to wiggle into, provided you choose the correct size (they extend from A to G). But they become increasingly laborious to stretch over a rear with each trip to the bathroom. By the end of a long workday, one could imagine fighting the urge to rip the damn things off and stash them in a purse. But Spanx, which have a cotton crotch, are actually meant to be worn every day in place of underwear. “It’s like a thong,” said Ms. Yalof Schwartz. “When you first put it on it’s very uncomfortable, but two weeks later you forget you’ve ever worn regular underwear.”

Of course, the impulse to tailor one’s natural proportions is nothing new. There were four centuries of corsetry, and girdles in the early 20th century—“even for the young and slim,” pointed out Valerie Steele, director of the museum at FIT—before we started letting it all hang out in the 1960’s. After women went to work in earnest, there were those crumpled control-top pantyhose in an egg, and then the fitness craze to keep us firm. We “internalized [girdles] through diet and exercise and surgical procedures like liposuction,” Ms. Steele mused. But maybe, just maybe, it all got too exhausting.

“Everyone’s so weight-obsessed in New York, it’s annoying,” said the socialite. “If people say, ‘Oh, you look so thin,’ I’m just like, ‘I’m wearing Spanx,’ and let it be.”

‘His Face Registered Horror’

Not everyone is in agreement about Spanx’s appeal.

“I was going to the 7th on Sale party, and I was supposed to wear a very slim-line jersey dress, vintage Bill Blass,” said Plum Sykes, the novelist, socialite and Vogue contributor. “The problem was it showed everything, this dress. So I thought, ‘Very well, I’ll go and get this thing called Spanx.’ I’m a little bony, so I wanted to reduce the boniness, but it looked awful!” Ms. Sykes faulted Spanx’s (very minor) seam down the front. “Those things can work; but I think you have to be more on the pudgy side, and then it sort of sucks you all in,” she concluded.

“I don’t feel very sexy when I have them on,” said the actress Chloë Sevigny. “They’re supposed to make you look good in pictures, but I’d rather feel good. I just suck it in.”

“I have a love/hate relationship with them,” said the socialite Fabiola Beracasa, Veronica Hearst’s daughter and the creative director of an estate jewelry company. “You’re always a little self-conscious about people touching you. People don’t just shake your hand and walk away. So you’re dancing around somebody because their arm is darting in your direction.”

Others embrace the protection of Spanx. “It makes me feel less naked,” said a Condé Nast fashion editor who asked not to be named. “Sometimes I get nervous that someone’s going to pinch my butt or brush by me, and it’s just going to be naked buttness!”

She suggested that those who find the neo-girdles uncomfortable are probably just wearing too small a size. “You don’t want to smash your butt; you just want to organize things.”

For all the wonders Spanx may work with that diaphanous Rodarte evening dress, however, there hovers a specter. Judy Woloshen, 30, a publicist for THINKFilm, said when she wears Spanx, she’s “praying that I don’t meet a gorgeous man who wants to rip off my dress.”

Who can forget the scenes in St. Elmo’s Fire and Bridget Jones’s Diary, of plump heroines being seduced and then summarily mocked by men who encounter their grandmotherly panties? Certainly not Nicolette, 26, a fashion designer who lives in the East Village and wore the Power Panties to a wedding under a cocktail dress. “This idiot I went to high school with was groping me on the dance floor,” she said. “He felt my abnormally hard ass, and his face registered horror, and he said, ‘What is this, some kind of girdle?’ I was humiliated. I seriously had to retire them.”

And yet some women have turned to wearing Spanx as a barrier against unwanted temptation. “I was out at Cain with a bunch of girlfriends,” said Rini, 27, who works at a hedge fund. “We ran into some Wall Street boys that I knew, and I ended up leaving with a particularly cute one. We started making out in the taxi, and then I realized I had Spanx on. I was so embarrassed and worried he would find out I was wearing a girdle that I wouldn’t let him touch me! I woke up grateful for my Spanx.”

Men are not so grateful (see sidebar). “I really detest Spanx,” said Jimmy Jellinek, former editor in chief of Maxim. “I feel like it’s a total bait and switch. You get it off and all of a sudden it’s like a fucking flab-alanche! It just comes out like a snow bank falling from a crevasse.”

When it comes down to it, maybe the only thing Spanx prove is that we dress for the beginning of the night, not the end. And their popularity shows no sign of waning. “I think there’s going to be an even larger audience, because dresses are getting tighter,” said Ms. Yalof Schwartz. “The baby-doll look is going to be done, if it’s not done already.”

Mr. Lawson suggests a compromise. “To me the next step is sexy Spanx,” he said. “Why can’t they have lace on them?”

Ah, who needs to be patronized by patriarchal lace? Kate S, the event planner, is satisfied with just plain Spanx. “A girl stands up a little straighter,” she said. “They give you Mannequin Ass. It doesn’t billow like it’s full of water. It just doesn’t move.”

She sighed. “They really are amazing.”