As she looks back, Janice can't believe that she and her husband, Steven, didn't figure out sooner what was wrong with their 13-year-old son. Kevin had always been a happy, sociable child who earned good grades. Now he was growing more emotional and exhausted by the day.

The Northern California couple (who asked us not to print their last name) took their son to a psychiatrist and a sleep-study center, but nothing helped. Kevin claimed to be too sick to go to school so many mornings that he missed most of eighth grade.

Janice, a nurse with a technical-writing background, and Steven, a software engineer, eventually discovered the problem: Kevin was spending 18 hours a day on one of the eight computers the family had scattered throughout the house. And he spent the bulk of that time playing an online game, fighting monsters, gathering equipment, and rising to much-coveted higher experience levels. "It became an addiction," Janice says.

David Greenfield, Ph.D, a clinical psychologist in West Hartford, Connecticut, has no doubt that Internet addiction is a real problem. The founder of the Center for Internet Studies, Greenfield cites a study of more than 17,000 users that suggests about 6 percent of all people with access to the Internet develop an addiction to it. That figure is even higher for kids and teens.

Like other substances and behaviors that can be addictive, such as drugs or even exercise, the Internet has the potential to alter a person's brain chemistry when he uses it, Greenfield says. When someone gets caught up in the thrill of winning an auction on Ebay, for example, or conquering opponents in an online game, the experience translates into a physiological response: the pulse quickens and the brain is flooded with pleasure-inducing chemicals. But just as users can become hooked on the physical rush of certain activities, they also can develop a tolerance, requiring more time online to get the same good feelings. And they can go through withdrawal if they lose their access. Greenfield relies on two criteria to help determine whether people are addicted to the Internet: They use it on a regular basis to alter their mood or consciousness, or it interferes with their life in any way.