After fielding a barrage of calls from regional managers this summer, Steve Keyes, human-resources officer at Nationwide Mutual Insurance, faced a quandary: what to do about exposed midriffs in the office.

Employees of the Columbus, Ohio, insurer were increasingly showing up for work in low-cut tank tops, flip-flops and blouses with spaghetti straps. Managers worried that this attire was creating an unprofessional work environment. So three weeks ago, Nationwide rewrote its dress code, banning its 35,000 employees from wearing midriff-baring tops, T-shirts and flip-flops. Mr. Keyes says the revision was necessary to keep up with changing styles. "Fashion trends impact the workplace," he says.

This summer's hottest fashions are taking casual to a new extreme -- and the result has been culture clash in many offices around the country. Though runway fashions are returning to a more prim look, mainstream retailers from Banana Republic to Intermix have been looking to capitalize on the success of high-end jeans by pushing other "dressy casual" styles. This has office workers showing up in knee-length city shorts, micro-miniskirts that rest just below the groin and a flood of new flip-flops. The new looks are happening just as many companies are returning to more formal attire after a decade of letting workers leave their suits at home.

Lazard Asset Management has had enough. On July 20, employees received an email with the subject line "Business Dress Policy." "We have generally allowed business casual attire during the hot summer months but, as a reminder, all employees are expected to maintain a professional, business-like appearance." It cited examples of "inappropriate casual dress" including "Spandex-type materials," sun dresses, halter tops, miniskirts, "casual sandals," flip-flops and hiking boots.

It's the same story at the U.S. Commerce Department, which earlier this summer sent out a dress-code reminder to its human-resources employees, warning against flip-flops, sweatshirts and jeans. In Chicago, real-estate developer Fifield Cos. says it's creating a new employee handbook this summer after a staffer wore a midriff-baring ensemble to a sales event.

The latest styles pose a particular challenge for employers because they aren't unequivocally casual. Indeed, many of the most popular styles in the past several years are built around juxtaposing formal pieces with inexpensive, laid-back elements. They're also intentionally designed to do double-duty and be worn for day and night -- something retailers believe appeals to younger shoppers in particular.

Human-resources officers are cracking down with stern and almost humorously specific lists of dos and don'ts that essentially turn them into fashion consultants. At Merlot Marketing in Sacramento, Calif., Chief Executive Debi Hammond found the question of city shorts came up so often she had to address it at a meeting. "People kept asking the office manager whether they could wear them," Ms. Hammond said of the dressy, knee-length shorts that are one of this summer's biggest fashion trends. The answer: yes, but only with closed-toe pumps and a dressy work shirt.

Harrison & Shriftman, a public-relations and event-planning company with offices in New York, Miami and Los Angeles, says it hasn't resorted to a formal dress code but gives some specific examples as guidance shortly after the new crop of summer interns arrived. For instance, short shorts are OK, but not when paired with a midriff-exposing or low cut shirt. Flip-flops, though allowed, should be "not your standard plastic," says principal Elizabeth Harrison.

At the Dubuque, Iowa, office of Prudential Retirement, employees used to spend hours debating whether capri pants -- which the dress policy permitted only on Fridays -- were actually the same thing as "cropped pants," which the policy allowed on any day.

"People would get in tiffs about whether they were capris or cropped pants," says Judy Lai, a financial analyst who worked in the Dubuque office. "Part of me kept thinking, 'We're going through a lot of stuff at work right now -- I can't believe we're wasting time on this kind of thing!'" The capri faction won: This summer, the office finally ruled that the pants could be worn any day of the week.

Emprise Bank in Wichita, Kan., which allows business-casual attire during the summer, says it was getting so many questions about what was acceptable earlier in the year, that it sent out a 20-page PowerPoint presentation to its 450 employees just before Memorial Day clarifying its guidelines. In one slide titled "How to Wear Crops at Emprise," a photograph of a woman wearing cropped pants and a blazer is captioned "Like this," while another shot showing the pants paired with an unbuttoned blouse says "Not like this." The company nixes untucked shirts and flip-flops for men, for example, while reminding them to "Be sure to use a belt when loops are available!" For women, open-toed shoes are OK but only if they're worn with pantyhose.

The clampdown isn't just a traditional battle against sloppiness or slacker employees who simply don't have the motivation to dress for success. Some of the looks that are causing friction right now are considered quite stylish. Those untucked shirts and rubber sandals, rather than indicating laziness, represent a studied casual look that many wearers go to lengths to achieve.

Annette Matlosz, a 25-year-old sales account executive for NationalCard Processing Systems in Rochelle Park, N.J., says she regularly wore city shorts, sleeveless blouses and skirts that fall above the knee this summer, despite her company's dress code, which encourages suits and jackets.

"To me, looking super cute is really important. It builds up my self-confidence and helps me have a better day," says Ms. Matlosz, who wore a lacy, see-through camisole blouse over an opaque tank top this past week, when temperatures hit 99 degrees. She says she's less able to do her sales job well in a suit because "I feel so bland -- I don't think I project well."

Ms. Matlosz says her goal is not to look like her parents: "I just remember when my father went to work, he would wear the exact same suit, the exact same tie every single day."

At Stanton Crenshaw Communications, a New York marketing firm, interns have recently shown up in ratty flip-flops, sheer tank tops and spaghetti straps, according to employees at the company. A young staffer in her 20s at Mercury Insurance in Sacramento, recently wore a skimpy blue tank top revealing a red bra underneath, according to one claims adjuster.

"I've had clients tell me that it adversely affects their productivity if they see others in the group dressing inappropriately," says Dr. Maynard Brusman, president of Working Resources, a workplace consulting company in San Francisco that works with high-tech, law and accounting firms. "It's wasting their time, they can't focus."

H&M, which sells inexpensive, trendy work clothes, says it nudges younger shoppers buying office attire toward "classic yet feminine" dresses and suits. "We suggest couture-inspired suit jackets and pencil skirts," says Jennifer Uglialoro, an H&M spokeswoman. "If shoppers are buying tank tops for work, they're usually throwing a blazer or jacket over it."

At Intermix, a chain of boutiques that's made its name selling fashion-forward designer clothing, CEO Khajak Keledjian says his sales associates push layering as "the perfect way to make a sexier piece work for the office." Mr. Keledjian, who currently has 13 stores in four states and the District of Columbia, says his sales people suggest wearing lightweight sweaters, cropped cardigans or fitted blazers over more flesh-flashing tops for the office and also steer shoppers toward fitted blouses with cap or short sleeves that are feminine without baring the shoulders.

Because suits, tailored clothing and formal attire in general tend to be more expensive, many retailers have been pushing the return of suits for several years. But while suit sales increased in 2004 and 2005, the category is now declining. In the 12 months ending in June, sales of men's and women's suits were $5.5 billion, a 13.8 percent decrease over the same time period a year earlier, according to NPD Group, a market-research firm in Port Washington, N.Y.

Business-casual dress codes began in the early 1990s. In 1991, Alcoa, the Pittsburgh-based aluminum giant, was one of the first firms to roll out a casual dress code when it allowed employees to dress casually for two weeks if they made a donation to United Way. The casual boom, which reached its peak with the dot-com craze of the late 1990s, has waned in recent years, with many businesses embracing a new workplace sobriety. In 2002, Lehman Brothers and Bear Stearns reinstituted a formal business dress code.

Today, some 84 percent of companies with 2,000 employees or more have a business-casual dress code, up from 79 percent two year ago, according to a survey of 1,400 companies by Mercer Human Resource Consulting. Joe Vocino, a senior consultant with Mercer, says the point of these policies is not so much to encourage business casual but rather to define the limits of what can be worn.

Bree Coven Brown, a 33-year-old writer for Mostad Media in Seattle, says she feels her generation has a devil-may-care fashion attitude precisely because of the dot-com boom. "We saw these super smart people who make a ton of money who could go to work with jeans and shirts," she says. "It gave permission to a whole generation of us to dress casually or outrageously at work."

The Dow Jones Office-Casual Risk Index

This summer's fashion trends are fueling a wave of extreme-casual clothes in the workplace. To find out which of the current styles can be pulled off in the office, we polled stylists, human-resources managers and CEOs

CITY SHORTS
Risk Factor: LOW
City shorts are appropriate office attire if you wear them as you would a skirt, says stylist Mary Walbridge. Pair them with a jacket and high heels and pick styles that fall just above the knee. American Express considers dressy city shorts "perfectly acceptable" in its New York corporate headquarters this summer, says Bet Franzone, a company spokesman.

MIDRIFF-BARING TOP
Risk Factor: SEVERE
CEOs and stylists agree: This look is not for the office. "Leave it for Britney Spears or Christina Aguilera," says New York stylist Sarah Shirley. Companies such as Nationwide Mutual Insurance have specifically banned the look in dress codes this summer.

MEN'S UNTUCKED DRESS SHIRTS
Risk Factor: ELEVATED
In any conservative office, shirts should be tucked in. But in some settings, such as Hollywood, Silicon Valley and design firms, a young man can pull off the look if he wears a high-quality, ironed shirt, says stylist Fati Parsia.

FLIP-FLOPS
Risk Factor: SEVERE
Employers from Northrop Grumman to the U.S. Commerce Department have banned flip-flops for men and women. Lori Erickson, vice president of human resources for Monster.com, says that in some cases, a dressier flip-flop style might pass at the office, but Joe Vocino, a senior consultant at Mercer Human Resource Consulting, says, "Never. Too casual."

MINISKIRTS
Risk factor: ELEVATED
Mr. Vocino says a skirt a few inches above the knee is "probably OK," but no higher. "Anything that's considered provocative is not acceptable," he adds. Stylist Lauren Chung advises sticking with hemlines no higher than two inches above the knee.

SPAGHETTI STRAPS
Risk Factor: ELEVATED
Shoulder-baring spaghetti straps can work only if they are covered by a jacket, says Ms. Shirley. Tank tops are acceptable only in the privacy of your office. Merlot Marketing Chief Executive Debi Hammond once sent a receptionist wearing a halter top home to change.

MEN'S T-SHIRTS
Risk Factor: SEVERE
Stay away from T-shirts completely at the office. Shirts with writing are particularly inappropriate. Ford Motor prohibits them entirely. Ms. Parsia advises avoiding them even in creative environments. "You'll be mistaken for the mailroom guy," she says.

SHEER BLOUSES
Risk Factor: ELEVATED
You can get away with a sheer blouse if you've got an opaque camisole on underneath and a jacket on top, says Ms. Chung. But wearing a brightly colored undergarment underneath won't pass muster at work.

SUNDRESS
Risk Factor: LOW
Tailored sundresses that hit the knee and have a conservative neckline can work if paired with a blazer. But it's a slippery slope. Dick Wolfe, an executive at Gibbs & Soell communications firm, nixed a job applicant for wearing a sundress he thought was too revealing and casual for a job interview.

CAPRI PANTS
Risk Factor: LOW
Many companies now allow capris, including Prudential Financial in Dubuque, Iowa, which recently decided that the cropped pants are acceptable five days a week, instead of just Fridays. Pair capris with pumps in a conservative office, or with ballet flats in a trendier environment, says Ms. Parsia.