Dr. David Greenfield isn't immune to the Internet's allure. When he was first hooked up at home four years ago, "I'd get online just to check my e-mail, and I'd stay on for two hours," he says. "It got to the point where I thought, If I'm experiencing this kind of pull, other people must be feeling it too.' "

Indeed they were. In a survey of nearly 18,000 Internet users that psychologist Greenfield, 43, author of Virtual Addiction: Help for Netheads, Cyberfreaks and Those Who Love Them, published on ABCNEWS.com, nearly 30 percent said they used the Net as a way to escape from problems including guilt, anxiety and depression. In other words, Greenfield contends, they're using it as a crutch—and that can be addictive. Though he calls the Internet "one of the greatest inventions of the 20th century," the Bronx-born Ph.D.—who holds degrees in psychology and counseling from Ramapo College, New York University and Texas Tech University-doesn't allow it to interfere with his nine-year marriage to fellow psychologist Marci Korwin or his relationship with their children Joshua, 5, and Jonathan, 3. "I have all the gadgets," he says, "but I know they're dangerous." He spoke with contributor Tom Duffy in West Hartford, Conn., where he works in private practice and lives.

How many people use the Internet?

Close to 100 million in the United States. And that's increasing daily by tens of thousands. E-mail and chat are by far the No. 1 and No. 2 uses. E-commerce sites come in third. And the sex-related sites are fourth.

Are there people who are spending too much time online?

Based on my research, about 6 percent of people online are using the Internet compulsively. Even if we've overestimated, we're talking millions. It's not that they just stay on for two hours. I'm talking about people losing jobs, having marital problems, experiencing a very significant negative impact on their lives. The average among the most compulsive group was upward of six to nine hours online a day.

Have you seen anything like this before?

This isn't a new disease. It's a new way of expressing the same disease: addiction. People get addicted to lots of things that are pleasurable and intense. The Internet gives you that hit, a temporary high feeling, just like exercise or drugs.

Can Internet shopping go too far?

All addictions are the same, regardless of the stimulus. You shop because you get a high. The problem is, it's so short-lived that you have to keep doing it. And that's where it can become a compulsive pattern.

What's compelling about the Net?

We don't know for sure. But people feel closer, quicker, to the people they communicate with online than in real life; time passes freely, and people like the anonymity. "With every other communication medium—newspapers, magazines, TV shows—there is a beginning and an end. But online, there is always another link, another banner, another person to answer that question. The Internet operates on the same reinforcement principle that gambling does: Every once in a while you are going to hit on something really good. It could be sex, information or that 40 bucks you just saved on a printer.

So how do people know when they've gone too far?

There are two things to ask: Are they using the Internet to alter their mood on a regular basis? And, is it interfering with their life in any way?

Is there a personality type that is more prone to compulsive use?

I don't know if Internet addicts are different from drug addicts or gambling addicts. We do know that they tend to be younger and there is a slightly higher incidence of addiction among people in the technical field.

What should people do if they use the Internet compulsively?

One solution is to limit the amount of time they spend online. Have a specific task you are going to do and write that down. Put a clock next to the screen so you can keep track of time. If you find yourself getting overstimulated by some site, limit your access to it. Ask someone to monitor your use or put the computer in the family room or the living room, where other people can see you. Try to beef up your real-time relationships to compete with your cyberfriendships. If you're shopping too much, go to a store instead. If you can't control your use, you might have to just log off.