For nine months, Riyadh Lafta, an Iraqi professor of medicine, tried to get a visa to visit the University of Washington, where he had been invited to share his research on the unusually high rates of cancer among children in southern Iraq.

But by last March, with no visa forthcoming, the American institution came up with an alternative plan. Mr. Lafta would deliver his lecture at Simon Fraser University, in Vancouver, British Columbia, and it would be broadcast by video to a public meeting long planned for the purpose at Washington.

The day before his mid-April flight, however, the British consulate in Amman, Jordan, turned down his request for a transit visa to change planes at London's Heathrow Airport. So Mr. Lafta, a faculty member at Baghdad's Al-Mustansiriya University, had to make the long and dangerous trip back to the Iraqi capital.

His American research partners say they think they know why he never received a U.S. visa: The Iraqi was one of the principal authors of an October 2006 study published in the British medical journal The Lancet that controversially estimated that more than 650,000 Iraqis — far more than officially reported — had died as a result of the American-led invasion.

Academic and civil-liberties groups say Mr. Lafta's case is troubling, but not unique. They assert that during the last year or so the Bush administration has increased its use of heightened security measures, introduced after the 2001 terrorist attacks, to keep out foreign scholars whose politics or ideas it does not like. In such cases the government does not give reasons for denying a visa, making it nearly impossible to challenge the decision, academic advocates say.

"Each new case seems to underscore the doubts that the administration has any justifiable security basis" to exclude the scholars, says Jonathan Knight, director of the program on academic freedom and tenure at the American Association of University Professors.

The pattern not only hurts the scholars in question, but also damages America's reputation for academic freedom, those groups say. Some academic associations have felt forced to move their meetings to Canada to ensure that members from other countries can attend. They also report that the United States has become a less appealing destination for foreign scholars.

"There are many people who simply don't think of teaching or attending a conference in the United States because they don't want to put up with the humiliation of the visa process," says Barbara Weinstein, president of the 14,000-member American Historical Association...