Switzerland agreed to hand over to the US details of 4,450 account holders
A client of the Swiss bank UBS has won an appeal against handing over data to US tax authorities.
A court ruled that failure to fill in a tax form did not constitute fraudulent behaviour, even if large sums of money were involved.
The test case challenges a deal made between Switzerland and the US, in which the Swiss agreed to hand over details of suspected tax avoiders.
A further 25 similar cases are still pending.
The Swiss cabinet will discuss next Wednesday how to make sure the Swiss-US tax agreement will be implemented.
UBS and the US Department of Justice have not commented on the decision.
Swiss tax authorities can still appeal against the court decision.
Last year, after a long-running dispute, Swiss authorities agreed to give the US a list of almost 4,500 names that the US Internal Revenue Service (IRS) suspects of using secret bank accounts to avoid tax.
The US accused UBS of hiding nearly $15bn in assets of US customers.