According to the regulator, the financial services provider is believed to have provided information of an estimated 2,000 UBS clients thought to have offshore accounts.
A further 2,450 names are expected to be handed over during the next few months, reports have claimed.
UBS agreed in August 2009 to provide the details as part of a settlement with US tax authorities after it was accused of helping US citizens evade paying tax.
In a statement, the IRS said: âBased on information received to date and assurances by the Swiss government, we anticipate being in a position to withdraw the John Doe summons this fall.â
All of the customer files are thought to have now been examined, the IRS added.
Figures published by UBS earlier in the year showed it recorded a pre-tax profit of 1.3 billion Swiss francs during the second quarter of 2010.
The figure was up on a loss of 1.85 billion Swiss francs seen in the same period last year.
By Jim Ottewill