Objective: The aim of this study was to compare the effect of methylphenidate (MPH) versus MPH + parent training in children with ADHD and oppositional defiant disorder/conduct disorder (ODD/CD) over a 12-month period. Method: After careful screening, 120 children diagnosed with ADHD + ODD/CD were included in the study. Treatment consisted of ongoing medication management for 12 months, with or without participation in a parent-training program beginning after the 1st month. Participants were not randomly assigned to treatment groups because of ethical, practical, and methodological reasons. Results: Data analyses revealed that mother–child relationship improvements and symptom severity did not benefit from parent training. Conclusion: The results of this study highlighted the positive role of MPH in ADHD. No significant effects were observed after the addition of parent training to MPH treatment. Clinicians should carefully follow patients’ improvements and titrate the MPH dosage during long-term treatment. (J. of Att. Dis. 2011; XX(X) 1-XX)