(FORTUNE Magazine) – EVEN IF YOU'VE BEEN FOLLOWING THE brouhaha over U.S. immigration policy closely, here's something you may not realize: Many of the most educated, skilled immigrants--including folks who have been working at U.S. companies for years --are going back to their home countries at the rate of about 1,000 a day. This foreign-born talent is an asset the U.S. can ill afford to lose. Consider that more than half of all Ph.D.s working in the U.S. are immigrants, as are 45% of physicists, computer scientists, and mathematicians. Meanwhile, about 40% of MIT grad students come from abroad.

"We're losing all this imported know-how when we're also not growing enough of our own," notes David Heenan, a former executive at Asian conglomerate Jardine Matheson, who has just spent five years studying the situation up close for his new book, Flight Capital: The Alarming Exodus of America's Best and Brightest (Davies-Black, $24.95). "For America's ability to compete, this has the makings of a 'perfect storm.'"

Why are so many superstars leaving? Heenan interviewed more than 100 luminaries like Charles Zhang, an MIT grad who went home to start Sohu.com, China's biggest web portal, and found that supercharged economic growth in their birth countries is luring expatriates home. Then, too, governments bent on bringing back their most prized native sons and daughters are offering everything from free rent to startup capital for new businesses. Beyond those economic incentives lie less tangible motives as well. A fair number of foreign-born superstars just get homesick. Many with young families told Heenan they wanted to raise their kids somewhere with better public schools and less MTV.

Obviously employers hoping to hold on to these folks can't singlehandedly change the schools, or the culture. Still, says Heenan, there are steps any company can take to entice its expats to stick around. "In many U.S. companies, immigrants look around and see that almost all the executives at the vice-president level and higher are homegrown American talent," he notes. "So they perceive that the only way to reach the top is to go home." One solution is to "look at career paths. Are there implicit barriers? Since so many immigrants are techies, it could be that stereotypes about tech employees in general are causing some talented people to get pigeonholed" in jobs for which they're overqualified. At the same time, he says, make sure that immigrants have mentors. That can help them feel more comfortable, not just at work, but in the U.S., which may be bewilderingly different from where they were raised.

What if, despite your best efforts, a foreign-born up-and-comer decides to pack his bags? Says Heenan: "When an expat announces he wants to go home, it's time to get creative." He suggests that companies create "dual citizenships," wherein highly valued employees put their skills to work in two markets. "You could have someone working for Microsoft or Intel or IBM in, say, both California and Beijing or Tel Aviv. You want to tap into their skills and ideas even after they've gone home to Delhi. Don't just do an exit interview and say good-bye." Unless, that is, you like the idea of competing against your former superstar on his home turf.