Aims To investigate the prevalence of pathological Internet use (PIU) and maladaptive Internet use (MIU) among adolescents in eleven European countries in relation to demographic, social factors and Internet accessibility. Design Cross-sectional survey Setting The 7th Framework EU funded project, Saving and Empowering Young Lives in Europe (SEYLE), is a randomized controlled trial (RCT) evaluating interventions for risk-behaviors among adolescents in Austria, Estonia, France, Germany, Hungary, Ireland, Israel, Italy, Romania, Slovenia and Spain with Sweden serving as the coordinating center. Participants 11,956 adolescents (F/M: 6731/5225; mean age: 14.9±0.89) recruited from randomly selected schools within the eleven study sites. Measurements Internet users were classified by gender into three categories: adaptive, maladaptive and pathological, based on the Young Diagnostic Questionnaire for Internet Addiction (YDQ). Findings The overall prevalence of PIU was 4.4%; higher among males than females (5.2% vs. 3.8%) and differed between countries (Χ2 =309.98; df=20; p<.001). PIU significantly correlated with mean hours online and male gender. Highest ranked online activities were watching videos, frequenting chat rooms and social networking; significantly higher rates of playing single-user games were found in males and social networking in females. Living in metropolitan areas was associated with PIU. Students not living with a biological parent, low-parental involvement and parental unemployment showed the highest relative risks of both MIU and PIU. Conclusions Across a range of countries in Europe, using the Young Diagnostic Questionnaire for Internet Addiction yields a prevalence of ′pathological internet use′ of 4.4% among adolescents, but varies by country and gender; adolescents lacking emotional and psychological support are at highest risk.