Objective: The aim of this study is to investigate differences in responses related to (mental) health and behavior between two methods of data collection: web-based (web) and paper-and-pencil (p&p). Study Design: Within each participating school all third-grade classes (mainly 14-15-year-old pupils) were randomly assigned to either the Internet condition (n = 271) or the paper-and-pencil condition (n = 261). Principal Findings: Significant but small differences were found for the strengths and difficulties subscales ’emotional symptoms’ (p&p > web) and ‘prosocial behavior’ (p&p > web), and carrying a weapon (web > p&p). Perceived level of privacy and confidentiality did not differ between the two modes. Conclusions: The findings suggest that in a controlled school setting, web-based administration of health indicators yields almost the same results as paper-and-pencil administration. To generalize these findings, we recommend repeated studies in other populations and settings.