In autumn 1995 The Norwegian Cancer Society in cooperation with The Research Center for Health Promotion, University of Bergen started a study of school-based interventions aiming at preventing smoking among pupils in Norwegian secondary schools. The study comprised a nationwide sample of 4441 students at 99 schools (195 classes). This panel of students is followed through annual data collections till they graduate in spring 1997. Written consensus from students and parents was obtained from 95%. Schools were systematically allocated to one of four groups: Group A, control; Group B, intervention, containing classroom program, involvement of parents and teacher courses; Group C, like B, but without teacher courses; Group D, like B, but without parental involvement. Baseline data were collected by questionnaires administered in class in November 1994 and the first follow-up survey was carried out in May 1995. At follow-up the proportion of smokers had increased by 8.3 percentage points in Group A (control) and by 1.9 percentage points in Group B (most extensive intervention). As expected, the recruitment of smokers was higher in Groups C and D than in the ideal intervention, but lower than in the control group. Effects of the most extensive program among subgroups of students were examined by comparing Groups A and B. Students are categorized as high risk or low risk based on scores on scales measuring sensation seeking, physical maturity, antisocial behavior and parental smoking. The effect of the program on recruitment of smokers seems to have been at least as strong or even stronger among 'high-risk' students than among other students.