Objective: To evaluate acceptability, feasibility and effectiveness of computer-tailored physical activity education among adolescents. Methods: Two classes of 7th graders from 10 randomly selected schools were assigned to the intervention (computer-tailored intervention, n = 139) or control (no-intervention, n = 142) condition. Questionnaires were completed 1 week before and 3 months after the intervention. The computer-tailored intervention was completed during classes. Results: Students had few problems with the diagnostic questions and with the use of a computer. About half of the students evaluated the advice as interesting and easy to understand and about 40% as personally relevant, easy to use and credible. Half of students evaluated the advice as too long and only 33% reported to have used the advice. The computer-tailored intervention was effective for increasing school related physical activity levels with on average 25 min per week (F = 3.4, P <.05), but not for increasing total physical activity or leisure time physical activity. Conclusions: A 1-h computer-tailored intervention offered during class-time has the potential to increase school related physical activity. Practice implications: Although favourable effects of large scale implementation may be expected, some adaptations that might increase effectiveness should be investigated in the future.