The young woman in the Esprit ad left the top button of her jeans open "because they were so tight I couldn't zip them up," giggles Laiko Bahrs, a shop manager in San Francisco and one of the models for the ad series.

The man and the woman wearing white jeans in the current Calvin Klein ads left their top buttons open "because it is something provocative, which we like," says Klein.

In a full-page ad for La Cetch, a model wears a jeans skirt with the top button undone "because she's wearing it backwards and that's the way it fit," explains Michael Hoffberg, president of the company.

It's hardly a coincidence that several companies are showing their pants partially unbuttoned these days. Wearing the waist button open on jeans has been around for years, and not just for those who have outgrown their pants. A discreet signal in the gay community, it is a popular look here at the Exile disco and is common among some groups in San Francisco. Male hookers on the Rue Ste. Anne in Paris often leave the top buttons of their jeans undone.

But the origin of the style is not entirely gay. When Levi's 501 button-fly jeans became popular in Europe in the late '70s, many kids bought them oversize -- sometimes because that was all they could get -- and wore them hiked up and cinched at the waist. The effect was like the top of a paper bag, and the waist button was usually left open to emphasize the look.

Designer Calvin Klein thinks wearing jeans with one button unbuttoned is just plain sexy, the effect he wanted in his new ads photographed in Puerto Vallarta, Mexico, by Bruce Weber. "I love the purity of the white- and cream-colored clothes. Particularly in contrast to the suggestiveness of the way they are worn with the open button," he says.

"With the button open, the fantasy starts," says style authority Robert L. Green. Michael Hoffberg, president of La Cetch jeans in Los Angeles, first showed unbuttoned jeans last month when he was featuring jeans with attached printed underwear. "The point then," he says, "was to let people know about the pants underneath."

Esprit stylist Margaret Nelson says that's really why she left the trousers unbuttoned in some of the new ads for that company. "There were these wonderful printed boxer shorts, and I just didn't want to show them on their own," she says. "But I never expect that anyone will walk around with their pants undone."

Doug Tompkins, president of the $825 million sportswear company, says he was surprised at the ads. "Those things start with kids on the street, not by a fashion house."

According to Tompkins, the pictures were not a big hit with the Esprit design staff, including Susie Tompkins, his wife. "They called it a cheapo trick," he says, but it was too late to reshoot the photographs.

"It is one of those little things that get in the wind," Tompkins adds. "It doesn't look so good to me -- it's not a nice look to me. It's an affectation that comes around and doesn't last long.

"At least I hope it won't."