Switzerland's authorities have told more than 500 clients of Swiss bank UBS that their account details are about to be given to US tax authorities.
They are the first of a list of almost 4,500 names that the US Internal Revenue Service (IRS) suspects of using secret bank accounts to avoid tax.
The move is the latest stage in a long-running wrangle between the Swiss authorities, UBS and the IRS.
The US accused UBS of hiding nearly $15bn in assets of US customers.
US authorities had been concerned that its citizens were using Switzerland's notoriously secretive banking system to avoid paying tax on income and assets.
The US Justice Department originally asked for the details of more than 50,000 accounts - something that was fiercely resisted by both the Swiss authorities and the bank, fearful that their reputation for discretion would be fatally undermined.
But UBS's position was weakened when it admitted it helped thousands of US clients use Swiss bank accounts to evade taxes. The three sides later agreed in August to concentrate on certain accounts.
Each of these 500 individuals, its believed, may have hidden money in a UBS account. But the Swiss authorities do not necessarily have concrete proof of tax evasion.
One reason for handing over details is simply if the account held more than one million Swiss francs and remained undisclosed between 2001 and 2008.
Clients have a month in which to appeal against the handover.