Adherence to attention deficit hyperactivity disorder (ADHD) treatment is important because, when untreated, it may have serious consequences with lifelong effects. In the case of adolescents on long-term medicine prescription, more knowledge is needed regarding adherence and factors influencing adherence, which was the purpose of this study. Adolescents (n = 101) on ADHD medication ≥6 months were administrated questionnaires at amonitoring appointment: Medication Adherence Report Scale (MARS), beliefs about medicines (BMQ) and the Brief Illness Perception Questionnaire (B-IPQ). Adherencewas high, the mean value was 88% of the maximum MARS score, and correlated positively with the "BMQ necessity-concerns differential" but negatively with "BMQ concerns"and "BMQ-side effects". Adolescents with more belief in the necessity of the medication, less concerns and less experience of side effects tended to be more adherent to medication prescription ("intentional non-adherence"),while "unintentional non-adherence" (forgetfulness) was associated with how much they perceived that their ADHD affected their lives. In a multiple regression model, the variance of MARS total (R2 = 0.21) and “intentional nonadherence” (R2 = 0.24) was explained by the “BMQ-necessity–concern differential” and “BMQ-experienced side effects”. The variance of “unintentional non-adherence” (R2 = 0.12) was explained by the “BMQ-necessity–concern differential” and “B-IPQ-consequences of ADHD”. In conclusion, adolescents on long-term medication reported good adherence, mainly influenced by more beliefs in the necessity versus concerns of the medications, less experienced side effects and more perceived consequences of ADHD. BMQ could be useful to identify risks of low adherence, which should be counteracted by partially gender-specific interventions.