Mae West will saunter into the offices of about 50 Boulder, Colo., companies this Friday.

Bonni deCaro is a sales manager for Dennis Printing Service and will be making her annual Halloween rounds. It is the sixth year Ms. deCaro has donned a costume to visit clients. This year's outfit is a black evening gown with a white feather boa, "strategically placed" sequins -- and a big blond wig.

"I can't be shy in this costume," says Ms. deCaro, who allows that she is in her late thirties. She will attempt Ms. West's trademark swagger and has chosen ten one-liners to fire off. Her two big concerns are whether the 175 gift bags she has put together for clients will be enough and whether her dress can withstand her climbing in and out of her car during her eight-hour odyssey.

Ms. deCaro's clients eagerly anticipate her visits. "Every year we take a picture with her -- it's a big thing," says Stefanie Pospischil, a marketing specialist at Proligo LLC, a biotechnology company based in Boulder.

Proligo is having its first company Halloween party that afternoon, including a costume contest, games and refreshments. About half of the 60 staffers at the company's headquarters are expected to wear a costume. Ms. Pospischil, 26 years old, says she will show up for work that day as Winnie the Pooh.

Halloween has become a big deal at many companies, as the holiday has grown in recent years well beyond a candy-fueled bacchanal for trick-or-treaters. More adults are dressing up these days, and many are wearing their costumes to work. A benefits survey by the Society for Human Resource Management in Alexandria, Va., found that 35% of employers had Halloween parties last year.

Workplace parties let workers mingle in a relaxed atmosphere and are good for morale. While year-end holiday parties often are more formal and can involve the dangerous elements of alcohol and gifts, Halloween also no doubt is popular at companies because it skirts most issues involving religious holidays.

"You'll probably offend the least number of people with a Halloween party," says John Putzier, a consultant and author of "Weirdos in the Workplace," a management guide.

Mr. Putzier advocates workplace programs that let employees express their creativity and have fun, because he believes such events can break down barriers that prevent effective collaboration.

The cautions and tips for making the most of workplace Halloween parties apply to all company social gatherings, says Richard Bayer, chief operating officer of the Five O'Clock Club, a New York career counseling and outplacement firm.

Topping his list: avoid drinking too much alcohol, which can impair your judgment. He also advises people to develop an "eight-word message," and have it ready in case a senior executive approaches and asks how you are. "It's a mistake to say, 'Just fine,' " Mr. Bayer says. Instead offer a quick example of a project you are working on that is going well. If the person follows up you can offer more details.

He also advises people to try to speak to as many groups of employees as possible. Such parties are an excellent opportunity to build your network, and to be seen in a positive light. You also can use the event to set up future meetings, Mr. Bayer says.

In some cases, letting your hair down at a company Halloween party, or other company event, can bring you closer to co-workers. Michelle Greene, a senior manager of corporate communications at SpectraLink, a manufacturer of wireless telephones for the workplace, recalls one male employee who did this literally on a recent Halloween party day. The "quiet, reserved" product-marketing staffer who was new to the company showed up wearing a yellow dress, a curly wig, and lipstick.

"That was the beginning of seeing him with a different sense of humor," she says. "It was a good ice breaker."

The Halloween party at SpectraLink, which also is based in Boulder, includes chili and corn-bread cook-offs with small cash prizes, says Ms. Greene, whose corn bread has taken first place before. Costumes are encouraged for the 225 employees at the company's headquarters. Ms. Greene, 36, says she wore a Cat Woman outfit for a past party.

Kevin Bethke, chief executive of Interactive Video Technologies, based in Marina del Rey, Calif., spent some time figuring out what sort of message he wanted to send his employees around Halloween. He created a video presentation that shows off the company's media platform, so employees will view his presentation on their desktop computers while he is vacationing in Finland this week.

What they see might surprise them. For the message, Mr. Bethke, 40, put on a scary mask and a black trench coat buttoned up to the neck, and he shined a red light on the right side of his face. With creepy organ music playing in the background, the presentation begins with the origin and history of Halloween, as narrated by "Professor Franken Stein of Transylvania University."

"It's pretty pathetic as an accent, but I tried," Mr. Bethke says. The presentation isn't just fun for the sake of fun, he says. He also discusses some company business, and notes that the presentation is "a perfect example of what our solution can do."