Valerie Sokolosky, a Dallas consultant who advises corporate executives on business etiquette, was getting ready to do a session for a top management retreat.

At the last minute, the CEO called. Could she, he wondered, "throw in a little bit about 'business casual'?"

There's little doubt about the growing popularity of "casual days" -- when some firms let employees leave the tie or heels at home. Research tells us that out of 500 large firms interviewed by the Society of Human Resources Management found that 67 percent had instituted casual days and that more than a third had made the change in the past several years.

Still, casual days are causing creases in some professionals' foreheads, if not in their slacks. Ambitious workers say they worry about making the right impression. Top executives say they are concerned about customers.

Consider, for instance, what happened to a young executive at the firm where Ms. Sokolosky was planning her management training.

The man was a new employee, and it was Friday. "At the last minute, some investment bankers flew in from Manhattan and wanted to have lunch. The CEO was prepared (with jacket and spare shirt in his office), and he was not. It was pretty embarrassing for him. After that, he said he'll always have something in the office he can grab," she said.

As in many decisions about corporate manners, the key is to look at the style of your company or industry, check out what those in the corporate hierarchy are doing and be adaptable," Ms. Sokolosky said.

Some companies try to head off problems by giving explicit guidelines about what is "business casual" and what's considered over the line.

Both Verizon's Telephone Operations headquarters in Irving and Foxmeyer Inc. in Carrollton set limits when they started casual Fridays this summer. Sandals, sneakers, shorts and jeans are taboo at both companies.

"Jeans have a wide range. The first thing you know, you'd have jeans with the knees cut out. Frayed jeans. They can get less than casual," said Richard Schaulin, vice president for human resources at Verizon. "To avoid that, we just stated it upfront."

At Southwest Airlines, however, spokeswoman Melanie Jones spent last Wednesday in shorts, sneakers, and a T-shirt. Southwest first started a "fun wear" promotion in 1986 to match its "fun fares." Since then, the airline's headquarters has gone to casual days, casual summers-- and, last February, casual year, she said.

The year-round policy was a reward to employees for achieving what Southwest dubs the "triple crown"-- the highest number of on-time arrivals and the least number of complaints about lost bags and bad service in the industry-- Ms. Jones said.

"Management's philosophy is that casual allows you to be more creative, to enjoy yourself more and therefore be more productive," Ms. Jones said. Still, she planned to wear a business suit for an out-of-town business trip the following day.

At Southwest Airlines, shorts may be fine, but it's generally better to err on the side of caution, said Derrill Osborn, director of men's clothing at Neiman Marcus.

"Some say casual Friday creates a more democratic environment. We say at Neiman Marcus, 'Don't be naive,'" Mr. Osborn said.

For executives, Mr. Osborn advises something like "the navy Giorgio Armani with a banded collar shirt, buttoned. It's a matter of not going to extremes," he said.

Ms. Sokolosky said that at that typical firm, khaki slacks and a sports shirt are fine for men. Put on a belt. Wear socks. And at minimum, keep a blazer in the closet.

For women, "It isn't sleeveless. It isn't jeans. It isn't sweatsuits," she said. Slacks in a loose cut, a skirt and blouse or longer-length dress shorts are safe casual-day wear for professional women, she said.

The managerial customer at Turtletique, a women's clothing store in Dallas, is choosing "a casual blazer and a T-shirt, with a neat belt and a neat pair of shoes, maybe socks, for her casual days," said store owner Nancy Diebolt. "She has an image she has to keep up."

Despite the worries and war stories, casual Fridays are working out well, say most executives at the companies that try them. Yes, there was the New Jersey pharmaceutical firm that called in Ms. Sokolosky to draw up parameters after headquarters staff members showed up in cycling pants.

At Verizon, the guidelines have been "generally observed," Mr. Schaulin said. In fact, casual Fridays have proved so popular with Verizon employees that the company is considering extending them beyond the summer.