A law President Bush signed in the runup to Independence Day had a patriotic ring to it: Tax breaks for U.S. troops and veterans, paid for by socking it to rich Americans who renounce their citizenship to sidestep the I.R.S.

But the Heroes Earnings Assistance and Relief Tax Act, known as the Heart Act for short, will also ensnare a less sinister population than American tax dodgers: well-to-do Canadians, Britons, Indians, and other foreign residents concluding long-term assignments in the U.S.

When these foreign bankers, software engineers, chemists, and others leave the country—if only to retire or transfer to a new posting abroad—the Heart Act, which was sponsored by Representative Charles Rangel, the New York Democrat, will tax them on the unrealized capital gains of their total global assets.

That includes supposedly tax-deferred U.S. retirement accounts, as well as assets like a cottage in Quebec, a share of a relative's business in Bangalore, or a great-grandmother's pearls kept in a London flat.

Henry Alden, president of the consulting firm Everest International Group, calls it "a very draconian tax with stunning liabilities."

Consider someone who'd paid $10,000 for a vacation home in France in 1980, came to the U.S. in 1990 when it was worth $100,000, and left the U.S. in 2008 when it was worth $1 million. That person would be subject to a capital gains tax of $135,000 on that one asset.

Only permanent residents, also known as green-card holders, will be stung. As a result, rich executives considering moving to the United States may increasingly select long-term visas, potentially depriving the country of wealthy immigrants. “Green cards are for poor people,” said Marnin Michaels, a partner in the Zurich office of the law firm Baker & McKenzie who advises wealthy clients.

Many developed countries have so-called exit taxes. But the law's critics say the Heart Act will, over time, tarnish the image of the U.S. as a friendly place for foreign talent and capital.

Congressional backers say that the law closes tax loopholes exploited by wealthy Americans who renounce their citizenship to evade paying taxes, and it applies to green-card holders to put all expatriates on the same footing. A spokesman for the House Ways and Means committee noted that both the House and Senate passed the law unanimously.

Concerned by several aspects of the law, Alden and other accountants who advise multinational companies are working behind the scenes to soften the rules before enforcement begins in earnest.

Alden, who serves on a task force for expatriate issues for the American Institute of Certified Public Accountants, said employers are just now considering the effects of the new rules.