Despite the particularly detrimental health risks of smoking for adolescent with asthma, several studies demonstrated higher smoking rates among asthmatic adolescents than among healthy adolescents. To gain insight into underlying mechanisms, longitudinal studies on differences in smoking predictors between asthmatic and non-asthmatic adolescents are essential. This longitudinal study with two waves with an 18 months interval tests the Theory of Planned Behavior (TPB) among 346 asthmatic adolescents and 3,733 non-asthmatic adolescents aged 12-16 years. Structural equation models were used to test the predictive value of the TPB in these two groups. The results show, consistent with the TPB, that smoking-related cognitions (attitude, perceived behavioral control, and subjective norm) predict smoking onset via intention among both asthmatic and non-asthmatic adolescents. The TPB predicted smoking onset even stronger among adolescents with asthma. These findings may contribute to the development of tailored interventions for the prevention of smoking among asthmatic adolescents.