This paper presents the short-term and long-term results of a randomized smoking prevention trial. The purpose was to evaluate two smoking prevention programs, a social influence (SI) program and a SI program with an additional decision-making component (SI(DM)). Moreover, the contribution of boosters was assessed as well. Fifty-two schools were randomly assigned to the SI program, the SI(DM) program or a control group. Half of the treatment schools were randomly assigned to the booster condition; the other half did not receive boosters. Both programs consisted of five lessons, each lasting 45 min, and were given in weekly sessions in grades 8 and 9 of high schools in the Netherlands. The most successful program was the SI program with boosters which resulted in a significantly lower increase in smoking rates (5.6 and 9.7%, respectively) compared to the control group (12.6 and 14.9%, respectively) at both 12 and 18 months follow-up. The results suggest that boosters can be an effective tool for maintaining or increasing the effectiveness of smoking prevention programs. It is recommended that the SI program with the booster be implemented at the national level, since this intervention showed the greatest behavioral effects.