The U.S. Treasury has removed the "use-it-or-lose-it" restriction of flexible spending accounts, but it's not clear whether the change will encourage more workers to take advantage of this way to stash away money tax-free to pay for health care expenses.

Right now, less than a quarter of U.S. workers have FSAs, according to Mercer, a leading benefits consultant. A major reason is that they run the risk of forfeiting the money they set aside unless they spend it in the same calendar year.

But in late October, the Treasury Department announced a new $500 rollover option that companies can adopt voluntarily.

A sore point. Employers are eager to offer rollover plans because leftover FSA money has been a sore point with employees for years, says Steve Jackson, senior vice president of third-party benefits administrator PrimePay.

Overall, some 14 million workers participate in FSA plans. Employees at big companies typically forfeit $60 per person in their FSA each year, says Mercer consultant Chris Renz. At smaller companies the rate is closer to $70.

'Grace period' caveat. One drawback to participation in the rollover option, though: It cannot be used in conjunction with the "grace period" that many employers offer to allow plan participants to spend leftover FSA money in the first quarter of the following year.

Nevertheless, benefits administrators expect a majority of companies to jump on board by 2015. They also expect an increase in the number of employees who participate and the amounts they contribute in pretax dollars.

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It's open enrollment season for benefits, and many companies are asking workers to decide how much they will contribute during 2014 to their FSAs, up to the $2,500 limit. Since the rule change was just announced, employees may not be able to find out in time if their company is going to adopt a rollover program, and when it will be effective.

"When I saw the bulletin, I thought great, but not now," says Shannon Swanson-Arend, director of benefits and wellness for New Brighton, Minn.-based APi Group Inc., parent of 40 fire-protection, industrial and construction companies.

The timing makes it hard to lure workers who were afraid of losing FSA money in the past. Indeed, about one in four do leave money on the table currently, says Alegeus Technologies, the largest provider of benefit administration services.

Benefits administrator HR Simplified found one client with about 9,000 employees had nearly half of those with an FSA lose money last year. As 94 percent of them lost less than $500, it shows the potential value of a rollover. The company is now considering adopting the rollover, perhaps even for the 2013 calendar year, says HR Simplified president Mike Melnychuk.

What to do in case of rollover. If your employer adopts a rollover for 2013, be prepared to spend your FSA account down to $500 fast, the way you would if you were up against the grace period deadline, says Mercer's Renz. The good news, he adds, is that this might be the last time you rush around buying extra eyeglasses to use up your funds before you lose them.