This paper evaluates a smoking prevention intervention aimed at vocational school students, consisting of an existing Dutch in‐school program (three lessons each lasting 50 min) and a computer‐based tailored out‐of‐school program (three tailored letters with smoking prevention messages mailed to students’ homes). Nineteen schools that already participated in the in‐school program were randomly assigned to the in‐school or to the combined in‐school and out‐of‐school condition. The remaining 17 schools were randomly assigned to the out‐of‐school condition or to the control group. Effect outcomes were assessed at 6, 12 and 18 months after a pre‐test, and were based on initiation among never‐smokers and continuation among ever‐smokers. Twelve months after the pre‐test (post‐test 2), the in‐school intervention was successful in preventing vocational school students from continuing to smoke, compared with students in the control condition [odds ratio (OR) = 0.49; 95% confidence interval (CI) = 0.29–0.84]. Eighteen months after the pre‐test (post‐test 3), the tailored out‐of‐school intervention was successful in preventing smoking initiation, compared with students in the control condition (OR = 0.42; 95% CI = 0.18–0.96). The effect of the combined approach was not larger than the sum of the effects of the in‐school and the out‐of‐school effects.