Starting in the late 1980s, the Social Security Administration received a flood of W-2 earnings reports with incorrect — sometimes simply fictitious — Social Security numbers. It stashed them in what it calls the "earnings suspense file" in the hope that someday it would figure out whom they belonged to.

The file has been mushrooming ever since: $189 billion worth of wages ended up recorded in the suspense file over the 1990s, two and a half times the amount of the 1980s.

In the current decade, the file is growing, on average, by more than $50 billion a year, generating $6 billion to $7 billion in Social Security tax revenue and about $1.5 billion in Medicare taxes.

In 2002 alone, the last year with figures released by the Social Security Administration, 9 million W-2s with incorrect Social Security numbers landed in the suspense file, accounting for $56 billion in earnings, or about 1.5 percent of total reported wages.

Social Security officials do not know what fraction of the suspense file corresponds to the earnings of illegal immigrants. But they suspect that the portion is significant.

"Our assumption is that about three-quarters of other-than-legal immigrants pay payroll taxes," said Stephen C. Goss, Social Security's chief actuary.

Other researchers say illegal immigrants are the main contributors to the suspense file. "Illegal immigrants account for the vast majority of the suspense file," said Nick Theodore, the director of the Center for Urban Economic Development at the University of Illinois at Chicago. "Especially its growth over the 1990s, as more and more undocumented immigrants entered the work force."

Using data from the Census Bureau's current population survey, Steven Camarota, director of research at the Center for Immigration Studies, an advocacy group in Washington that favors more limits on immigration, estimated that 3.8 million households headed by illegal immigrants generated $6.4 billion in Social Security taxes in 2002.