WALNUT CREEK, Calif. - As Congress negotiates its biggest immigration overhaul in decades, new numbers obtained by the Contra Costa Times reveal a stunning imbalance in the program that admits highly skilled immigrants to the United States, often for Silicon Valley jobs. More than 70 percent of those special visa holders who entered the country in 2011 were men.

The long-overlooked disparity is beginning to attract attention on Capitol Hill, where activists demanded Monday that the federal government take a closer look at whether U.S. visa policy discriminates against women.

The numbers are especially striking because women now outnumber men in America's professional workforce, although they continue to lag in the engineering professions that make up a large number of the H-1B program for temporary immigrants.

"More men are coming simply because companies prefer to hire the men over the women," said Karen Panetta, a Tufts University computer engineering professor who called attention to the gap Monday at a hearing in the U.S. Senate. Panetta was testifying on behalf of the Institute of Electrical and Electronic Engineers, an association that represents thousands of San Francisco Bay Area and Silicon Valley workers.

Corporate hiring practices, outdated U.S. visa policies and entrenched gender discrimination in immigrants' home countries are all contributing to the disparity. The hearing marked the first time this year that lawmakers specifically addressed how reform of the immigration system will affect women.

While the Obama administration came under fire at the hearing for not revealing how many men and women hold H-1B visas, the nation's centerpiece program for highly skilled workers, the data requested by the Bay Area News Group provided the scope of the imbalance: The U.S. Office of Immigration Statistics recorded 347,087 male H-1B visa holders entered the country during the 2011 fiscal year compared with 137,522 women.

The data are imperfect because they include many H-1B immigrants traveling to the United States after visits to their home countries, not just first-time arrivals.

But the H-1B divide tells only part of the story of immigrant women's plight in the American workplace.

Among professional and management workers, about 67,000 immigrant men and only 39,000 immigrant women earned green cards last year for permanent U.S. residency, a 63 percent-37 percent gender imbalance in a country where women hold 51.5 percent of professional and management jobs, according to annual visa statistics and the Department of Labor.

"More men are coming simply because companies prefer to hire the men over the women."

Karen Panetta, Tufts University computer engineering professor