Nancy Mathis, a spokeswoman for the I.R.S., said the agency did not know the reason for the increase, but added, “It is likely due to more effective outreach by community organizations and proposed legislation last year linking payment of taxes to eventual citizenship.”

In 2005 alone, more than $5 billion in tax liability — the total owed, including money withheld from paychecks during the year — was reported in the 2.9 million returns that listed at least one person with an ITIN, she said. And between 1996 and 2003, such filers reported nearly $50 billion of tax liability.

Organizations pushing for stricter immigration enforcement have criticized the use of the individual numbers by illegal immigrants, saying that allowing them to file returns, pay taxes and receive refund checks legitimizes their illegal presence. But the I.R.S. says it is just doing its job as a tax collector, and is not an immigration enforcement agency.

“Clearly, we maintain a separation between the two systems,” Mark W. Everson, the commissioner of the I.R.S., said recently. “We want your money whether you are here legally or not and whether you earned it legally or not.”

And because most immigration bills make the payment of back taxes a prerequisite for legal status, that message seems to be getting through more clearly than ever.

“Right now there’s quite a stampede going on in many areas, including mine,” said Blaire Borthayre, a consultant in Hispanic marketing based in Raleigh who described early-morning crowds gathering outside Family Tax Service, her husband’s tax preparation business. “It’s word of mouth.”

One client, she said, was a Mexican man who lugged in a suitcase holding $14,000, cash he had set aside to pay his taxes over six years of work as a landscaper. Like many, he had only recently learned that he could file a legitimate tax return, said Ms. Borthayre, who is the author of several books on tax preparation with ITINs.