Objectives: To identify the factors affecting long-term adherence to methylphenidate treatment in children with attention-deficit hyperactivity disorder (ADHD). Methods: A retrospective medical record review of 239 ADHD patients (mean age $9.3{pm}2.6$ years, range 6.0-17.4 years) who had visited the child and adolescent psychiatry clinic at a university hospital, in Seoul, Korea from March 2005 to February 2008. Subjects were diagnosed as ADHD based on the criteria set forth in the Diagnostic and Statistical Manual of Mental Disorders 4th edition, text revision version (DSM-IV-TR) and underwent neuropsychological tests including the continuous performance test (CPT). Treatment discontinuation was defined as the last prescription date when the medication possession rate (MPR) became less than 0.80. Subjects were divided into three groups and labeled as Group I, non-adherence without pharmacotherapy, Group II, non-adherence with short-term pharmacotherapy, and Group III, adherence with long-term pharmacotherapy. Results: Ninety (37.7%) patients were grouped as non-adherent (Groups I+II) and 149 (62.3%) as adherent (Group III). The adherence group exhibited lower intelligence, higher symptom severity, and a higher number of comorbid psychiatric disorders than controls. The use of stimulants was significantly associated with long-term adherence to treatment. Additionally, the duration of interval between the date of the first visit and the date of the first prescription was positively associated with long-term adherence. Conclusion: About two-thirds of patients diagnosed as ADHD adhered to the treatment six months after the first visit. With respect to patient evaluation and the development of treatment strategies, factors affecting early drop-out and longer follow-up must be considered.