You're putting in more hours than ever. Think you're impressing the boss?Think again. Here's why some employers are forcing staffers out the door.

MARY ELLEN KENNY is the kind of employee bosses dream about. She's the type who, in a crunch, will work 20 hours straight, go home just long enough to shower and hop right back to her office at New York-based advertising giant Young & Rubicam for another day of pitch meetings.

Still, everybody needs to recharge their batteries now and again, and so Kenny finally planned a two-week Hawaiian getaway-her first time off in years. But just days before she was to jet away with her fiance, Kenny tallied up everything that needed to be done and decided she couldn't afford to be away for a single minute.

"I have to cancel this vacation," she told her boss, Linda Srere, then head of Kenny's group and now Y&R president.

That's when Srere — a strong believer in the virtues of vacations — got tough. "You have to go," she told her. "You've had this booked forever."

Kenny refused, so Srere got even tougher. First, she phoned Kenny's fiance and told him not to cancel the plans. Then she gathered her staff the Friday before Kenny was scheduled to depart. "Once she leaves, no one take her phone calls or answer her emails," Srere decreed. Finally, she called in Kenny. "If you come in on Monday, the locks will be changed for the next two weeks," Srere told her. "So you might as well get on that plane."

Some companies are turning that philosophy into policy. Take Stephen Schuetz, a managing partner of Ernst & Young's audit practice, who delivered a directive during a staff meeting: If his underlings don't take vacations, he'll haul them into his office and demand an explanation. Or former Salomon Smith Barney chairman Jamie Dimon, who at a weekend gathering of top performers called out the name of one broker who hadn't taken time off in six years, told him to stand up in front of the crowd of 500 — and badgered him to take a vacation, pronto. (He did.)

Even with such edicts, employees don't always listen, and so bosses have to engage in old-fashioned trickery. That's what happened to Diane Strahan. As vice president of marketing for job search Web site CareerBuilder.com, she averages 14-hour days. She's so busy she didn't get around to planning her tenth anniversary in May, let alone take time off for it. So Chief Executive Rob McGovern got deceptive. He had Strahan's assistant book phantom meetings for her the Friday and Monday around her anniversary weekend.

When that Friday rolled around, Strahan showed up at her office and was shocked to find her husband there, card in hand. "You'll be boarding a plane in two hours to Amelia Island, Fla.," Strahan read out loud, as her staff congregated behind her. A romantic scene, to which Strahan touchingly replied: "Are you crazy? I've got two full days of meetings."

Her boss then revealed the ruse. Strahan, still railing about how much work she had to do, was whisked to the airport by her hubby.

Skimping on time off has long been something of an American tradition, and employers share much of the blame. On average, one-year employees get only 9.4 vacation days. Compare that with the mandated 32 in Sweden. Still, a quarter of U.S. workers don't take the full time they do have coming, according to a study by the Families and Work Institute.

What feeds this? Often, it's fear. "They have these irrational feelings that they're going to be fired and lose everything," says Renana Brooks, a Washington, D.C., psychologist who works with high achievers. "It's too anxiety-provoking for them to delegate and see that other people can handle things." Adds Young & Rubicam's Kenny: "You feel like you're going to let down the team. You're afraid everything's going to fall apart without you."

Kenny did indeed miss something big that came up while she was diving among the corals of Kauai: a battle for United Airlines' advertising account. Her firm ended up landing the international segment, worth $70 million. But did Kenny's absence hurt her? Consider her current title: senior partner...for the United Airlines account.