Blakely in Spanx shapewear. Photograph by Josef Astor

One sticky morning last summer, Sara Blakely, the inventor of Spanx, which over the past decade has become to women’s foundation garments what Scotch is to cellophane tape, was sitting in the Park Avenue offices of her husband, Jesse Itzler, confronting a new challenge: the male anatomy. Red boxes of stretchy Spanx undershirts for men were strewn across a table before the couple. “Sara sent my dad, who is going to be eighty-two years old, a tank top,” Itzler said.

Blakely smiled. “He said it took him half a day to get into it, and half a day to get out,” she said. “My mom said she’d never laughed harder,” Itzler added.

There was also a prototype of black cotton briefs with a sturdy “3D” pouch over the groin, devised by Spanx’s product-development team after several male testers complained to Blakely that they needed more support. “They were, like, ‘It just kind of hangs,’ ” she said.

“And is the hole big enough? To get through?” Blakely went on, fingering the pouch. “It seems like you’d have to really . . . bring it out. Look, I’m in foreign territory here.”

“Yes, yes,” Itzler said, rolling his eyes.

Blakely, who recently turned forty and is a size 6 (“the largest I’ve ever been,” she said), with long blond hair and bright-white teeth, believes that there is no figure problem—saddlebags, upper-arm jiggle, stomach rolls—that can’t be solved with a little judiciously placed Lycra. “Where I get my energy is: ‘How can I make it better?’ ” Blakely said. “I’ll ask my brother, ‘If you could wave your wand and make your boxer shorts better, what would you do?” Her first big idea, in 1998, was to chop the feet off a pair of control-top panty hose so that she could get a svelte, seamless look under white slacks without stockings poking out of her sandals. The resulting product, Footless Pantyhose, has sold nine million pairs since October of 2000, when Blakely, who was then a fax-machine saleswoman and a part-time standup comic, started Spanx, with five thousand dollars of savings. Another Spanx product, a lightweight girdle called Power Panties that retails for around thirty dollars, has sold six million units since it was introduced, in 2002.

Before Spanx, shapewear was associated with the aging and the piteous (remember Hugh Grant pawing at Renée Zellweger’s thickly swaddled nether regions in “Bridget Jones’s Diary”?). Now unmentionables are not just mentionable but objects of boasting. Oprah Winfrey, with her ongoing weight struggles, has been a big booster all along, but admitting that you are wearing Spanx under your evening gown has become standard red-carpet patter even for slender celebrities. (The company encourages these testimonials by sending starlets regular complimentary shipments.) Gwyneth Paltrow is an outspoken fan, and the famously callipygian television personality Kim Kardashian makes frequent requests for free Spanx. “It just really smooths everything out,” she told me.

Itzler, who is tall and athletic and has curly blond hair, is a former rapper who wrote the nineties Knicks jingle “Go, New York, Go.” He currently owns a marketing and “business incubation” company called Suite 850, and he was proposing a cake-box package for a holiday promotional mailing of men’s Spanx to his influential friends, including Adam Sandler, LeBron James, and Gisele Bündchen, the supermodel, who is married to the Patriots quarterback Tom Brady. “I like the concept of ‘have your cake and eat it, too,’ ” Itzler said.

“You could also do ‘Happy Spanxgiving,’ ” Blakely said, jiggling her left leg; she had arisen early with the couple’s twenty-month-old son, Lazer, and drunk a large chai latte. “ ‘Stuff yourself, don’t worry about it.’ ”

After the meeting, Itzler strode out of the room on neon-soled sneakers to prepare for a flight to Boston, where he had some business involving Zico, a coconut water. Blakely got herself a can of Coke Zero.

“Mah husband is such a healthy eater,” she said. Spanx is based in Atlanta, and Blakely, who was brought up in Clearwater, Florida, tends to slip into a drawl when discussing domestic matters. “Except when it comes to sweets. He never consumes anything except fruit until noon. And then from noon on he might have some brown rice and some tofu, and then, come eight or nine at night, he orders three mud-pie double-chocolate pieces of cake and eats all three of them.”

Spanx’s popularity repudiates the late-twentieth-century belief, perpetuated by Jane Fonda and Nike, that a firm body can be achieved only through sweaty resolve. “There’s a whole subset of women who don’t relate to that idea,” Blakely said. She is overseeing a new line of activewear, called In It to Slim It, but there is a desultory feel to the enterprise. “I started thinking about joy,” Blakely said. “Everything in our society is so purposeful. Let’s bring joy back to the experience—have fun when you’re doing it,” meaning exercise. She has already expanded into legwear (Tight-End Tights); lacy lingerie (Haute Contour); casual separates (Bod a Bing!); and retro, ruffled swimwear. Spanx now offers more than two hundred different products, and executives at the company, which is privately held, and reported three hundred and fifty million dollars in global retail sales in 2008, worry that customers are having trouble distinguishing among them. Part of the line is manufactured by Acme-McCrary, a century-old firm in the hosiery mecca of Asheboro, North Carolina (the rest is outsourced to other countries). Larry Small, who until recently was Acme-McCrary’s C.E.O., told me that Spanx represents close to a third of his business, and he called Blakely a “rock star” in an industry of good ol’ boys. “I’ve always wondered how the heck men are supposed to sell hosiery,” he said.

Blakely chose the brand’s name partly for what she calls its “virgin-whore tension,” and partly for its “k” sound, which has a good track record in both business and comedy. “I used to hold my breath every time I said it out loud,” she told me. “People were so offended they’d hang up on me.” When the Spanx Web site first went live, Blakely’s mother accidentally directed a tableful of luncheon guests to spanks.com, a porn site.

The line is sold in thirty countries, and at more than ten thousand retail locations in the United States, including Saks Fifth Avenue and Neiman Marcus, where Blakely seduced her first buyer by pulling a sample of the original Footless out of a red nylon backpack—“My friends were, like, ‘Sara, buy a Prada bag,’ ” she said—and dragging her into the ladies’ room for a demonstration. Blakely’s creations are also available at Target, under the name Assets; and on QVC, where, during a recent appearance, she sold ninety-eight thousand Slimplicity camisoles, offering “super” compression, in a twenty-four-hour period. (“God bless America,” she said.) Around 3 A.M. last July 20th, a company milestone of sorts was passed when a woman was caught on a surveillance video robbing the cash register of a McDonald’s in Midwest City, Oklahoma, with a white girdle from Spanx’s Slim Cognito line pulled over her head.

Though Blakely often appears in public as the face, and the rear end, of Spanx, she has features of the sort that are hard to fix in memory. She has the slightly daffy energy of a cheerleader, a sorority sister, or an imaginative babysitter, all of which she used to be. “I feel like I’m standing on the edge of being a lot more famous,” she told me. Once, a cashier at Target asked Blakely why she looked so familiar. “Mind you, there’s an enormous Spanx store display with my picture on it,” Blakely said. “So finally I go, ‘I invented Spanx.’ She’s, like, ‘No, that’s not how I know you.’ I start listing my entire résumé: ‘I was on Richard Branson’s TV show. I was a judge on ‘American Inventor.’ After all that, she pauses and says, ‘You look just like my realtor.’ ”