The next trade show you attend may be following your every move.

Like a lot of trade show exhibitors, Kimberly Foley never gets enough information about the hordes of people cruising through her display. Foley, director of event marketing for Philips Healthcare, which spends several million dollars a year on trade shows, set up a 31,000-square-foot exhibit at a radiology event in Chicago last year. Philips filled it with big-ticket medical equipment like magnetic resonance imagers, CT scanners and X-ray devices. The show's 62,000 attendees, half of them health care professionals, were her target audience, but she never knew who was browsing and who was ready to buy.

Last year her intelligence got a whole lot better when the show organizers agreed to outfit each attendee with a name tag that includes a paper-thin wireless chip encoded with a 24-digit number. Twenty-five scanners in the Philips booth logged 9,600 visitors over five days. Alliance Tech, the Austin, Tex. firm that makes the wireless tags and the scanners that read them, provided Foley with job title, employer and specialty for each of those visitors.

"We don't get exact contact info," says Foley, "but we can tell if it is a Mayo Clinic doctor who visited 22 times and spent ten hours in the booth. We use it to figure out what they're most interested in."

This year Foley is doubling the coverage to 50 scanners, at a cost of some $50,000. She's even considered adding plasma screens that are networked to the scanners and offer content tailored to specific interests. For instance, when a radiologist walks up to the exhibit, the display might say, "Hello, Dr. Smith. We would like to show you some images in the reading room."

Alliance Tech claims its system is the first radio-frequency identification (RFID) system to track trade show and conference-goers. RFID has been around since World War II and was used to tell if an approaching plane was friend or foe. Wal-Mart (nyse: WMT - news - people ) uses it to track pallets of diapers and toothpaste in some distribution centers and stores. It was only a matter of time before Big Brother, or Big Marketer, started tracking people. Since last year all new U.S. passports have been embedded with close-range RFID. The Beijing Olympics organizers printed RFID tags on 14 million tickets that were read by 1,000 readers. That project reportedly cost $7.2 million.

Alliance Tech's founders, Roger Lewis, 48, Timothy Blackwood, 46, and Arturo Borrego, 44, get lots of questions about invasion of privacy but insist that their system avoids snooping by storing any personal information in a separate database. They can provide specific names, but many customers don't even ask, lest the trade show visitor be spooked off. And they say people don't mind being tracked. "We always offer attendees the chance to opt out if they don't want their name to show up," says Borrego, the company chief executive. "But over the past four years the number of people opting out has fallen from 8% to 1%."

That is, if they have a choice. Cisco (nasdaq: CSCO - news - people ) uses RFID to track employee attendance at training sessions; Deloitte and Grant Thornton have plans to. In 2005 IBM used RFID to track 12,000 salespeople sent to Las Vegas for training sessions held in 160 rooms at the Venetian and Mirage hotels. "They want to make sure employees are not running off and gambling," says Lewis, Alliance Tech's executive vice president of sales. The touch screens on Alliance's scanners can reveal someone trying to sneak in with two tags, one for a colleague off at the craps table.

"It's not like Big Blue is watching you," says Rebecca Swanick, a marketing director at IBM's Rational software division, who uses the technology at trade shows. "We need to track attendees without a lot of man-hours. In the past we had to eyeball it."

Before starting Alliance Tech six years ago, Lewis, Blackwood and Borrego worked together at IBM. Finding tags that could be scanned on people was a new challenge. Metal and liquids (e.g., a bottle of water) interfered with the communication. They finally settled on longer, rectangular tags ($1 each) and Motorola (nyse: MOT - news - people ) scanners that can register each tag 34 times as it passes through an entranceway.

Alliance Tech's scanners rent for $1,000 each for the course of a convention. The firm grossed $3.2 million last year, up from $2.5 million in 2006. Lewis plans to grow by selling to shopping malls. Frequent spenders might want to wear tags in high-end malls that would trigger text messages and videos about sales as they walk the mall. Invasion of privacy pays.

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