Adolescent smokers engage in cognitive rationalisation processes that lower perceptions of personal vulnerability to the health consequences of smoking. There is, however, hardly any evidence that provides in-depth insights on adolescents’ recurring rationalisations. Therefore, we explored how boy smokers deal with the knowledge that they are engaging in potentially fatal behaviour. Interviews were held with 16 boy smokers aged 16 to 17 years old. The qualitative analysis focussed on combining boys’ reasons about why they think they will not experience severe health consequences into coherent rationalisations that recurred among interviewees. Three rationalisations emerged from the analysis. First, boys trivialize the immediate consequences and think these can be compensated for and are outweighed by the benefits of smoking. Second, boys assume that smoking will only take place during adolescence and they will, therefore, recover from the damage inflicted. Third, boys believe that they have control over the amount and frequency of smoking and, thereby, can ensure that they will not experience fatal consequences. Boys’ recurring rationalisations build on their view that they are supposed to have fun and will not become typical adult smokers. Interventions should address these rationalisations in order to increase adolescents’ perceptions of personal vulnerability, and thereby contribute to decreasing adolescent smoking.