More immigrants came to the United States illegally from 2000 through 2004 than the number who were granted legal status in those years, according to a study released yesterday that attributed much of the historic shift to visa slowdowns since 2001 and to the nation's strong job market before that.

The study by the Pew Hispanic Center said that immigration to the United States -- legal and illegal, from all regions of the world -- totaled about 1.1 million each year during the 1990s, peaked in 2000 at 1.5 million and declined substantially since 2001 to earlier levels. The number of new arrivals increased in 2004, the study said, though it is too early to say that the rise will last.

But the study, based mainly on Census Bureau surveys, said legal immigration rose more slowly during the 1990s and declined more rapidly since 2000 than did illegal immigration. One result, it said, is that the 562,000 estimated new illegal immigrants who arrived last year was about the same as a decade earlier, while the number of new legal immigrants, an estimated 455,000, was lower.

Pew demographer Jeffrey S. Passel said he believes this was the first time in the nation's history that new illegal arrivals outnumbered new legal immigrants. "The presence of the undocumented makes a big difference," he said. "This is what differentiates this from 100 years ago. There really wasn't anything like what we call illegal immigration today."

The study made the point that although the foreign-born population rises each year, now numbering at least 34 million, the pace of new arrivals fluctuates for a variety of reasons. "The flow of migration is not this inexorable, constant increase," said Roberto Suro, the center's director.

The nation's buoyant job market in the late 1990s fueled the influx of new illegal immigrants and new temporary migrants such as high-tech workers, according to Passel, and both groups declined in number when the economy slowed. The study's total of illegal immigrants includes a small number of people with temporary papers who are seeking permanent visas.

Meanwhile, he said, the government reduced the number of refugee visas and slowed the processing of other visas after the 2001 terrorist attacks, which contributed to the decline in legal immigration. This affected people in the United States with temporary papers who apply for green cards, as well as those who hope to come to the United States.

A spokesman for the U.S. Citizenship and Immigration Services, the government agency that oversees the granting of green cards, said he could not comment on the study in detail because officials had not examined it closely. But Bill Strassberger questioned its conclusion that new illegal immigrants outnumber legal ones.

"I don't know that there are any indications that the undocumented population is growing faster than the legal population," the spokesman said.

Some demographers say that family and village networks are so entrenched that immigration will rise despite the economy. Steven A. Camarota, research director for the Center for Immigration Studies, said there may have been "some falloff" in immigration since 2001, but there are not enough data to say there has been a substantial drop linked to the economy. Camarota, who supports limits on immigration, agrees with the Pew study that the number of illegal immigrants is growing rapidly.

The report did not break out totals for the Washington area, one of the most popular destinations for new immigrants, but Passel said the region's strong economy probably dampened any immigration decline since 2001.