WASHINGTON — Two years after they became legal, President Bush has begun to champion health savings accounts as a salve for the nation's ailing healthcare system, proposing $156 billion in tax breaks to encourage Americans to buy an unorthodox kind of insurance that is favored by conservatives but whose merits are largely unproven.

Early studies of HSAs — and the early experiences of a small but growing number of people who are trying them — do not match the White House's certainty that this recent concept in health insurance is, as Bush put it recently, "good for you."

Health savings accounts differ sharply from traditional insurance by requiring people to pay more of their own medical expenses in exchange for significant tax benefits if they set aside money for that purpose. The arrangement consists of two parts: an insurance policy — less expensive than most ordinary health plans — in which people pay at least a few thousand dollars up front before the coverage begins, combined with a special investment account into which they and sometimes their employers may save money tax-free for current or future medical expenses.

According to the White House and other proponents, the plans can tame medical costs, turn patients into smarter medical consumers and make insurance affordable for more people. HSAs, however, remain so new and rare that there is little evidence on whether they curb overall health- care expenditures or overuse of care. Meanwhile, research hints that they are most appealing to people who are relatively affluent, not poor and uninsured.

Some people who have switched to the plans are delighted. "So far, it's been very, very good for myself and my family," said Daniel Reisfield, 47, a Wall Street headhunter who lives in San Diego with his wife and two children. Reisfield said he thinks it makes more sense to invest more than $5,000 a year in mutual funds through his HSA than to pay a large insurance premium for his healthy family, which rarely seeks medical care.

Others have been disenchanted quickly. Felix Meschke, 32, a business professor at the University of Minnesota, did careful calculations of medical probabilities before he, too, switched his family to an HSA on Jan. 1. Less than two weeks later, his 11-month-old son, Jason, developed an ear infection that progressed to a fever and persistent cough so worrisome that a pediatrician sent him to a hospital, where he stayed for two days. By Jan. 24, Jason was getting better, but Meschke faced a $3,700 bill. If he still had his old insurance, he would have owed a few hundred dollars.

The uneven, uncertain impact of HSAs on the broader landscape of healthcare can be glimpsed at Wendy's International Inc. The fast-food chain has moved as aggressively as any corporation to switch to health savings plans, and Bush chose its Ohio headquarters as his destination for a recent speech to talk up HSAs. "They're good for Wendy's," he said. "They'll be good for you, as well."

Last year, Wendy's eliminated its old insurance plan for about 9,000 managers and administrators and offered them only an HSA instead. The company has not offered such a plan to its hamburger cooks and the rest of its front-line crew, most of whom do not work enough hours or are not in the right parts of the country to qualify for health insurance — and tend not to buy it, even if they qualify.

"We think HSAs are the prudent way for people to insure themselves in America," said Allan Hubbard, director of the White House's National Economic Council.

Urged on by business, the banking industry and conservatives in Congress, the White House is defining HSAs as part of what Bush has called an "ownership society" that shifts responsibility — and, critics say, risk — from government and employers to individuals. The budget the president recently recommended to Congress includes three tax breaks, totaling $156 billion over the next 10 years, to encourage the use of health savings plans.

The plans became legal in 2004 under a bitterly contested aspect of a Medicare law. House conservatives insisted on allowing HSAs in exchange for supporting an expensive new drug benefit for older Americans; many Senate Republicans were less enthusiastic, and most Democrats fought the idea. The critics say HSAs mainly provide a tax break for people with good incomes and health, and create a dangerous ripple effect in which traditional insurance eventually would cost more for everyone else.

During the past two years, about 3 million Americans, out of 170 million with private insurance, have started to try them, according to insurance industry figures. Bush has said repeatedly that a third of people with such a plan were uninsured beforehand, though two industry surveys suggest that is an overstatement. And though Bush has said that two-fifths of families with HSAs earn less than $50,000 a year, research and some companies' experience suggest the plans are most attractive to people who have relatively large salaries; people with modest incomes who have HSAs tend to be at small companies that do not provide a choice, early studies suggest. Unlike Wendy's, most of the big employers that are trying HSAs offer them as one alternative, and they are much less popular.

At the defense contractor Raytheon Co., for instance, about 1,000 of its 67,000 workers picked an HSA when the accounts became available at the beginning of this year. Rose Umile, senior manager of employee benefits, said the company offers to chip in about a third of the contributions to the health accounts, is fully covering preventive care and encourages employees to think of the plans as a way to get a head start on saving money for medical care once they retire. The result, Umile said, was that "higher-paid employees purchased it."

When Impinj Inc., a semiconductor company based in Seattle, offered HSAs a year ago, Autumn West tried one. Last month, she returned to a traditional health plan. A 30-year-old single mother with a salary of about $60,000 as a human resources assistant, West uses more than $500 a month worth of medicine for severe allergies and asthma and needs treatments that her HSA plan counted as surgery, requiring her to pay a larger share. Even with Impinj chipping in most of her $3,000 deductible, she still found herself paying more than she used to. "It hasn't worked out as well as I would have liked it to," she said.

Several of West's co-workers at Impinj are happier with their new HSAs. One of them, Magdalene Adenau, who is 29 and single, ended last year with about $700 left in her health account. And when she needed a chiropractor, she went to one who charged about $60 until she shopped around and found one who charged about $30. "There's a real benefit to you if you use the money smartly," Adenau said.

But when his son was hospitalized, said Meschke, the Minnesota professor, "I realized that I neither had the bargaining power nor mental capacity" to influence the price of Jason's chest X-rays, intravenous fluids or antibiotics. The hospital staff he asked about the charges had no idea, he said, "and you are kind of overwhelmed with the medical aspects of this. If you're negotiating a car, you can always say, 'I'll walk off the lot.' If your 1-year-old kid has an I-V in his arm, you don't have the same situation."

A survey last fall by the Employee Benefits Research Institute found that people with HSAs were more likely than those with other health plans to delay or avoid care when they were sick.

Plan more limited in its coverage

Health savings accounts have two parts: a high-deductible health plan and a tax-free savings account.

The health plans are more limited than regular coverage. For 2006, they require people to pay a deductible of at least $1,050 for individuals and $2,100 for families before the coverage begins.