The Rotterdam-based Institute for Research into Lifestyles and Addiction (IVO) surveyed 512 young people aged between 13 and 15 and found that what it termed "compulsive internet use" was indicative of an increase in feelings of depression and a decline in school results within six months.
The IVO found that Internet "addicts" spent on average 26 hours a week in front of the computer terminal.
Parents had the largest role in preventing Internet addiction, the study found, suggesting the parents should talk to their offspring about it if they saw a problem looming.
"How often this happens is not as important as making clear to the young people that their feelings are understood and offering them support," the IVO said.
It suggested that setting clear - but flexible - rules regarding Internet use helped to prevent addiction.
"If young people in this age group are allowed very little computer time, this appears to have a counterproductive effect and boosts compulsive Internet behaviour," the IVO said.