Hundreds of thousands of Internet users show compulsion for porn sites, X-rated chat rooms, or other sexual materials online. In one of the first studies to estimate the number of "cybersex compulsives,'' researchers say it is at least 200,000, and millions more are at risk.

"This is a hidden public health hazard exploding, in part, because very few are recognizing it as such or taking it seriously," say psychologists at Stanford and Duquesne universities.

The researchers classified users as "cybersex compulsives" if they spent more than 11 hours a week visiting sexually-oriented areas and scored high on a 10-item questionnaire about relationships and attitudes toward sex.

The researchers, led by Al Cooper, clinical director at the San Jose Marital and Sexuality Centre, found evidence that compulsives have more problems with relationships and jobs than Internet users who visit X-rated sites casually. "There is and should be a lot of concerns about these people," Cooper said. "They are developing problems that can be serious."