OSTON -- ABOUT 10,000 people a week go to The Rack, a bar in Boston favored by sports stars, including members of the New England Patriots. One by one, they hand over their driver's licenses to a doorman, who swipes them through a sleek black machine. If a license is valid and its holder is over 21, a red light blinks and the patron is waved through.

But most of the customers are not aware that it also pulls up the name, address, birth date and other personal details from a data strip on the back of the license. Even height, eye color and sometimes Social Security number are registered.

"You swipe the license, and all of a sudden someone's whole life as we know it pops up in front of you," said Paul Barclay, the bar's owner. "It's almost voyeuristic."

Mr. Barclay bought the machine to keep out underage drinkers who use fake ID's. But he soon found that he could build a database of personal information, providing an intimate perspective on his clientele that can be useful in marketing. "It's not just an ID check," he said. "It's a tool."

Now, for any given night or hour, he can break down his clientele by sex, age, ZIP code or other characteristics. If he wanted to, he could find out how many blond women named Karen over 5 feet 2 inches came in over a weekend, or how many of his customers have the middle initial M. More practically, he can build mailing lists based on all that data — and keep track of who comes back.

Bar codes and other tracking mechanisms have become one of the most powerful forces in automating and analyzing product inventory and sales over the last three decades. Now, in a trend that alarms privacy advocates, the approach is being applied to people through the simple driver's license, carried by more than 90 percent of American adults.

Already, about 40 states issue driver's licenses with bar codes or magnetic stripes that carry standardized data, and most of the others plan to issue them within the next few years.

Scanners that can read the licenses are slowly proliferating across the country. So far the machines have been most popular with bars and convenience stores, which use them to thwart underage purchasers of alcohol and cigarettes.

In response to the terrorist attacks last year, scanners are now also being installed as security devices in airports, hospitals and government buildings. Many other businesses — drugstores and other stores, car- rental agencies and casinos among them — are expressing interest in the technology.

The devices have already proved useful for law enforcement. Police departments have called bars to see if certain names and Social Security numbers show up on their customer lists.

The electronic trails created by scanning driver's licenses are raising concerns among privacy advocates. Standards and scanning, they say, are a dangerous combination that essentially creates a de facto national identity card or internal passport that can be registered in many databases.

"Function creep is a primary rule of databases and identifiers," said Barry Steinhardt, associate director of the American Civil Liberties Union, citing how the Social Security number, originally meant for old-age benefits, has become a universal identifier for financial and other transactions. "History teaches us that even if protections are incorporated in the first place, they don't stay in place for long."

But companies that market the scanning technology argue that it poses no threat to privacy.

"It's the same information as the front of the license," said Frank Mandelbaum, chairman and chief executive of Intelli- Check, a manufacturer of license-scanning equipment based in Woodbury, N.Y. "If I were to go into a bar and they had a photocopier, they could photocopy the license or they could write it down. They are not giving us any information that violates privacy."

Machine-readable driver's licenses have been introduced over the last decade under standards set by the American Association of Motor Vehicle Administrators, an umbrella group of state officials.

Under current standards, the magnetic stripe and bar codes essentially contain the same information that is on the front of the driver's licenses. In addition to name, address and birth date, the machine-readable data includes physical attributes like sex, height, weight, hair color, eye color and whether corrective lenses are required. Some states that put the driver's Social Security number on the license also store it on the data strip.

The scanning systems present a challenge to efforts by state and federal governments to limit the amount of information that can be released by departments of motor vehicles. In 1994, Congress passed the Driver's Privacy Protection Act, largely in response to the murder of Rebecca Schaeffer, an actress who was killed in 1989 by an obsessed fan who had found her unlisted address by using California motor vehicle records.

Before the law was adopted, states were selling driver's license information to direct marketing companies, charities and political campaigns. Businesses selling, for example, fitness products and plus-size clothing were able to focus on customers within a given range of height or weight.

While the privacy act staunched the flow of information from state motor vehicle departments, there are only spotty controls over how businesses can create such databases on their own. In Texas, the driver's licenses can be electronically scanned for age verification, but the information cannot be downloaded from the machine. In New York, businesses are only allowed to store name, birth date, driver's license ID number and expiration date for the purpose of age verification. Many states require people to give consent to be on marketing lists, but businesses generally interpret consent to mean not actively removing their names from a list.

When Mr. Barclay, the bar owner, saw a demonstration of Intelli-Check (news/quote)'s driver's license scanner at a trade show in 1999, he was surprised. "It had never dawned me that that strip had information on it," he said.