We evaluated the association between those symptoms/behaviours of attention-deficit/hyperactivity disorder (ADHD) that were present at diagnosis and outcomes of treatment in children and adolescents in six European countries. Physicians abstracted clinical records from patients (6-17 years) diagnosed with ADHD between 2004 and 2007 and treated for ≥2 years. Physicians scored the severity of impairment for core ADHD symptoms and additional (non-core) ADHD symptoms/behaviours at diagnosis and estimated treatment adherence (defined as an estimated >80 % adherence on weekdays and >50 % adherence on weekends). Treatment modalities included pharmacological treatment, behavioural therapy, or both. Pharmacological treatment was further subclassified by medication class. The outcome, optimal treatment success (OTS), was defined as complete symptom control with high satisfaction with treatment. Multivariate logistic regression modelling examined the relationship between OTS and symptom impairment. Of 730 patients, 200 (27 %) achieved OTS. These patients were more likely to demonstrate lower impairment in non-core ADHD symptoms/behaviours and have fewer pre-existing comorbidities. They were also more likely to be adherent and engaged with treatment, with an explicit treatment goal to improve inattention/school performance. Neither core symptoms' severity nor treatment types were associated with OTS. OTS rates were low, with patients having less impairment of non-core ADHD symptoms/behaviours and fewer comorbidities more likely to achieve OTS. Potentially modifiable factors affecting OTS were as follows: treatment adherence, treatment engagement, and a treatment goal to improve inattention/school performance. These data suggest that there may be opportunities to optimize current treatment use, and develop new treatment strategies to improve core and non-core ADHD symptoms/behaviours.