Objective: Treatment adherence is an important aspect of ADHD symptom management, but there are many factors that may influence adherence. Method: This analysis assessed adherence to OROS methylphenidate during a 1-year, open-label study in children. Adherence was defined as the number of days medication was taken divided by the number of days in the study and determined to be high if greater than or equal to 75%. Possible clinical and demographic factors associated with adherence, including use of planned medication breaks, were assessed. Results: Mean adherence was 86.4%. It was 91.6% for the subgroup of patients who reported not taking planned medication breaks (n = 252) and 77.7% for the subgroup taking planned medication breaks (n = 155). Overall, 75% of patients showed high adherence. Older age, low starting dose, minority ethnic status, and fewer ADHD symptoms were associated with low adherence. Conclusion: Various factors were found to be associated with low adherence, and the results of this analysis provide guidance to physicians seeking to identify those patients with ADHD most likely not be adherent to stimulant therapy. (Contains 6 tables and 4 notes.)