A new study of census data concludes that recent immigrants were critical to the nation's economic growth in the past decade, accounting for half of the new wage earners who joined the labor force in those years.

The effect was particularly large among men: Eight of 10 new male workers in the decade were immigrants who arrived during that time, according to the report by the Center for Labor Market Studies at Northeastern University.

New immigrants accounted for 76 percent of the labor force growth in Maryland and 44 percent in Virginia. In the District, where the workforce declined, immigrants prevented further shrinkage.

The Northeastern University report, scheduled for release this week, offers powerful new evidence of the growing impact of immigrants in American society. Earlier data from the 2000 Census showed a record number of new arrivals during the 1990s that prevented population loss in some cities and rural areas. The newly analyzed workforce numbers show that immigration also is redrawing the profile of the U.S. workforce, in some cases transforming entire industries.

More than 13 million immigrants came to the United States from 1990 to 2001 -- some legally and some illegally -- drawn by the healthy economy and family ties. The report said 8 million immigrants joined the labor force, which means they were either working or looking for work, over a period when the total number of new workers was 16 million.

The impact on the workforce was significantly larger than in the previous decades. In the 1970s, for example, immigrants accounted for 10 percent of the labor force growth. It increased to roughly a quarter in the '80s before expanding to half in the '90s.

Even so, 86 percent of the workforce is American-born.

For decades, the nation's immigration policy has been a subject of intense debate, with critics saying the large numbers strain schools and other government services and take jobs from American-born workers. One of the authors of the Northeastern study argues that the research indicates the opposite: The U.S. economy would have stumbled in the past decade without the new arrivals, and most immigrants contribute more in taxes than they use in services.

"The American economy absolutely needs immigrants," said Andrew Sum, director of the labor market center. "I realize some workers have been hurt by this, and some people get very angry when I say this, but our economy has become more dependent on immigrant labor than at any time in the last 100 years."

Sum said many of the new immigrant workers, possibly half, are here without legal papers, meaning that the immigrants have an uncertain future and that the economy is dependent on people in a legal no man's land.

The center's report was commissioned by the Business Roundtable, a group of corporate chief executives.

A study released last week underscores that the immigration trend seemingly is here to stay, absent a drastic change in policy. The Center for Immigration Studies in Washington, which favors limits on immigration, said that even the economic downturn and a crackdown on illegal entry since the 2001 terrorist attacks have barely pinched the pace of arrivals. Two million immigrants have arrived in the United States since the 2000 Census, according to Census Bureau survey data quoted in the center's report.

The Baltimore-Washington region gained 175,000 new immigrants in the two years since the 2000 Census, the report said. That represents an escalation of the pace over the previous decade.

Susan Traiman, director of the workforce education initiative at the Business Roundtable, said the Northeastern report sheds new light on the underpinnings of the vigorous '90s economy. "We would not have been able to have this economic growth without the growth in the workforce that was supplied by immigrants," she said.

More than a third of the new immigrants were employed in blue-collar occupations, but nearly one in four held a technical, managerial or professional job. They were concentrated in certain sectors, especially manufacturing, retail trade, business and repair services, and personal and entertainment services. But they also have an above-average share of the nation's jobs in engineering, computer science and physical science.

The impact of new immigrant workers varied by age group and region. Without new immigrants, the labor force would have experienced no growth in New England and the New York region. In the fast-growing southern and Rocky Mountain states, however, which drew population from elsewhere in the country, immigrants had less of an effect.

Immigrants also accounted for all the growth among workers under 35. That is explained by a drop in U.S. birthrates in the 1970s and the resulting dip in the U.S.-born population in that young age group. But even among those ages 35 to 44 -- the youngest baby boomers -- new immigrants supplied a third of the growth in the labor force.

Anirban Basu, chief economist of the Regional Economic Studies Institute at Towson University, said the arrival of immigrants "helped fill in some gaps that would have otherwise persisted in the labor force," especially in the high-tech sector and among younger workers.

"Not only did immigrants add to the raw total of the number of workers, but they added in very meaningful ways," he said.

The effect was particularly noticeable among male workers, in part because of a puzzling decades-long decline in the share of U.S.-born men in the workforce. One factor is early retirements; another is that male high school dropouts are less likely to work than they were in the past.

Had it not been for immigrants, the report said, "the nation's entire male labor force would have grown only marginally over the past decade, and male labor shortages would likely have been widespread in many areas of the country, especially the Northeast and Pacific regions."

Among women, three in 10 new workers were recent immigrants, a much smaller proportion than among men. That's because U.S.-born women are continuing to enter the workforce in larger numbers, the report points out, while immigrant women are much less likely to work.

The report cited evidence that the entry of many poorly educated immigrants into the workforce has held back wages of the lowest-paid American-born workers. And Sum said U.S.-born workers can be shunted aside when the economy slackens because employers often prefer to hire immigrants, believing that they work harder.

Nine in 10 new immigrants went to work for private industry, having a profound effect on some companies. At the 7-Eleven nationwide convenience store chain, for example, a heavily immigrant workforce allowed aggressive expansion in the 1990s, when the company added several hundred stores each year, spokeswoman Margaret Chabris said.

Its increasingly foreign-born workforce required the company to adapt. In mid-decade, 7-Eleven added a unit to its training program to "teach the nuances of providing good customer service in America," she said. "We teach foreign-born employees from certain cultures that American customers want to be looked in the eye. They want their change handed to them rather than being put on the counter."

The Northeastern University report was based on a broader definition of immigrant than the government uses, including not only people born in foreign countries but also those from Puerto Rico, the U.S. Virgin Islands and other U.S. island territories. People born in those territories accounted for only 368,000 of the 13.5 million people deemed immigrants in the report. The workforce numbers were for civilians only, not the military.

Korean immigrants Lisa and Peter Lim work at Montgomery Custom Tailor in Bethesda, one of their two tailor shops.