Purpose: The purpose of this paper is to focus on the evaluation of an internet-delivered stress-prevention program for adolescents as a possible alternative for school-based implementation of mental health promotion. Design/methodology/approach: A total of 904 adolescents in grades eight and nine were assigned to four treatment conditions (online-training in school, online-training via internet from home, school-based face-to-face training, control group without intervention). Before and after the training interval, all adolescents were questioned about their knowledge regarding stress and coping and their appraisal of stress-evoking situations. The participants self-assessed their perceived stress vulnerability, their coping behavior and their stress symptoms. In addition, the training groups were asked about their training acceptance. Findings: The results show a considerable knowledge gain for participants of the online-program. The number of positive cognitions in stress-evoking situations also increased, although this effect was only observed in the school-based online-training setting. Regarding training acceptance, the best results were obtained for participants of the classical face-to-face intervention. Research limitations/implications: Future research should examine factors that influence compliance rates in internet-delivered prevention programs. Practical implications: Although online-prevention cannot completely substitute school-based face-to-face-training, it can be seen as an effective and economic complement to conventional methods of health promotion. Originality/value: This paper presents not only a comparative evaluation of internet- and school-based health promotion targeting adolescents, but also a comparison of the effects of online-prevention under "real" and "ideal" conditions. (Contains 4 tables and 1 figure.)