At least we're doing better than in 2006, when the figure was four unused days per worker. The Harris/Expedia research last spring, among a nationwide cross-section of more than 4,000 American workers, asked about vacation plans for the year.

Averaging the number of days left behind per worker would be meaningful if everyone who works in the United States actually got vacations. But we don't. About 75 percent of the work force gets paid vacation — an average of 14 days, plus whatever holidays employers grant under labor agreements or by fiat.

Not only do we regularly give up days we've often bargained hard to get, we also get few compared with the rest of the industrialized world. In fact, the United States is singular when it comes to vacation days: We are the only advanced economy in the world without a minute of government-mandated time off.

In order to be a member of the European Union, a nation's employers must offer workers a minimum of 20 days off a year. Several mandate more. According to "No-Vacation Nation," a study by the Center for Economic and Policy Research about the United States' unique place in the balance between work and lifestyle, France requires the most vacation: 30 days, plus a paid holiday. Overall, though, the French are beaten by Austria and Portugal, which require employers to give 22 vacation days off, plus 13 holidays, for a total of 35 days off — amounting to seven weeks a year.

Among the richest countries, Canada and Japan mandate the fewest days off, not counting the U.S. total of zero. The Japanese, who have a word — karoshi — for dying from overwork, get a minimum of 10 days and no nationally mandated holidays. Canadians have the same minimum, but get an additional eight paid holidays.

According to people who research employee benefits, workers in those countries also leave time behind. The Harris/Expedia survey says the French also give up three days, the Spanish, two, and the Germans, one — but they all have more time off, and an incentive to use it because it's more of a cultural mandate.

Workaholism. The Puritan ethic. Feeling guilty about taking earned days. The fear of being overrun by competition. People frequently cite these reasons for junking their vacation days.

Others, often at the worker-bee level, save vacation days for family emergencies or appointments that crop up. "If the cable guy is going to come by the house, a lot of people have to take a vacation to be there, for instance," says John Schmitt, coauthor of the "No-Vacation Nation" study. Or, says Bernard N. Katz, of the employment law firm Meranze & Katz in Philadelphia, "people will try to save vacation days for supplemental sick leave, for when they can't work."

And clearly, a number of people who fail to take their time off really like their work. After cramming in more than a week of unused vacation time at the last minute, Christiana Brenner, a senior account executive with a Chicago firm, ended 2007 with 11 unused vacation days. Brenner, 26, says she's "fortunate to have a job I truly enjoy" and even considers fun.

Last year, Cassandra Oryl didn't give up any of her 10 vacation days and three holidays, but the year before, she traveled for fun to Portugal, and still left days behind at Braithwaite Communications, a Rittenhouse Square public relations firm. "Because we're an agency, we ultimately have to answer to clients," she says. "So some years, there just isn't much time for vacation."