Although the efficacy and tolerability of ADHD medications have been investigated fairly extensively, there are very few data comparing the different types of medication (e.g. psychostimulants, non-stimulants) in terms of medication adherence. The primary research objective of the COMPLY observational study was to evaluate medication adherence (i.e. compliance) over 1 year in children and adolescents with ADHD in a routine clinical setting. COMPLY was a prospective 12-month, observational, open-label study that included children and adolescents, aged 6-17 years, with ADHD. Medication adherence (i.e. compliance) was measured using the Pediatric Compliance Self-Rating (PCSR) instrument and using items 1-4 of the Medication Adherence Rating Scale (MARS). A total of 504 patients were enrolled. At baseline, 252 patients (50.0 %) were prescribed non-stimulant (atomoxetine) medication and 247 patients (49.0 %) were prescribed psychostimulant medication. Both types of medication were prescribed concomitantly in five patients (1.0 %). After 12 months, 123 patients (48.8 %) were taking atomoxetine and 176 patients (71.3 %) were taking psychostimulants. Adherence (PCSR score ≥ 5) was present in both groups (atomoxetine: 67.5 %; psychostimulant: 74.2 %) throughout the observation period. MARS scores declined over time in both groups (atomoxetine: from 3.7 to 2.9; psychostimulant: from 3.6 to 3.1), indicating a deterioration in adherence. There was no statistically significant difference in terms of medication adherence between the two groups.