Background: There is a lack of effective smoking cessation interventions which have a high reach among adolescent smokers. Method: We conducted a randomised controlled trial of an adolescent smoking cessation intervention for students aged 15-21 at 22 continuation schools in Denmark. The intervention was a minimal intervention based on events with short counselling and a range of self-help materials. Our 1-month follow up consisted of 514 baseline daily smokers and the 14-month follow up of 369 daily smokers. Results: We found positive short-term effects regarding smoking cessation (adjusted OR 4.50, 95% CI 1.20-16.86), self-efficacy (p=0.01), and intentions to quit (p=0.004). However, none of these effects were maintained at 1-year follow up. The intervention was successful in reaching more than half of all daily smokers in the target group with more than 30% receiving counselling at least once in the intervention period. Conclusions: This trial shows that a "minimal" intervention can produce short-term effects but that this approach is insufficient in producing long-term effects.