AIMS: The aim of the present study was to examine the association between social-cognitive factors, school factors, and smoking initiation among adolescents who had never smoked. METHODS: The study was based on longitudinal data on Danish adolescents attending randomly selected public schools. Adolescents enrolled in grade 7 (mean age, 13 years) who had never smoked (n = 912) were followed up for 6 months after baseline. Those who had still never smoked were followed up again 18 months after baseline, in grade 8 (n = 442). Social-cognitive factors were examined with five measures: self-efficacy, social influence (norms), social influence (behavior), social influence (pressure), and attitude. We used multilevel analyses to estimate the associations between social-cognitive factors at baseline and smoking initiation as well as the random effects of school, school class, and gender group in the school class. RESULTS: At the first follow-up, we found significant associations between attitude, father's smoking, best friend's smoking, and smoking initiation. At the second follow-up, we found a significant association with pressure to smoke from friends. Of the school factors, gender group in the school class showed an effect at both first and second follow-up. CONCLUSION: Our results suggest that father's smoking, best friend's smoking, attitude, and pressure to smoke from friends affect smoking initiation. The results for school factors suggest an effect of classmates of the same gender, which has not previously been examined longitudinally.