BACKGROUND: The objectives of the study were to develop web-based Spanish and Catalan versions of the KIDSCREEN, and to compare scores and psychometric properties with the paper version. METHODS: Internet and paper Spanish and Catalan versions of the KIDSCREEN-52 were included in a cross-sectional study in school-age children. Web-based and paper Spanish or Catalan versions of the KIDSCREEN-52 were administered to students aged 8 to 18 years from primary and secondary schools in Palafolls (Barcelona, Spain, n = 923). All students completed both web-based and paper versions during school time with an interval of at least 2 hours between administrations. The order of administration was randomized. The KIDSCREEN-52, the Strengths and Difficulties Questionnaire (SDQ), and sociodemographic variables were collected. Missing values, floor and ceiling effects, and internal consistency were compared between both versions, as well as mean score differences, level of agreement, and known groups and construct validity. RESULTS: Participation rate was 77% (n = 715). Web-based and paper versions showed low percentage of missing values and similar high ceiling effect (range 0 to 44%). Mean score differences showed an effect size (ES) lower than 0.2 in all dimensions. Internal consistency ranged from 0.7 to 0.88, and degree of agreement was excellent (Intraclass correlation coefficient [ICC] range 0.75 to 0.87). Expected differences were seen by sex, age, socioeconomic status and mental health status. CONCLUSIONS: The web-based KIDSCREEN-52 showed similar scale score and reliability and validity than the paper version. It will incorporate the child population in the assessment of quality of life providing a more attractive format.