Objective: Oppositional defiant disorder (ODD) is the most common comorbid condition in childhood ADHD. This trial was prospectively designed to explore ODD symptoms in ADHD adults. Method: A total of 86 patients in this placebo-controlled, double-blind trial of methylphenidate transdermal system (MTS) were categorized based on the presence of ODD symptoms in childhood and adulthood, and then were compared for baseline and outcome differences. Results: In all, 42% met "Diagnostic and Statistical Manual of Mental Disorders" (4th ed.; "DSM-IV") criteria for ODD as adults and were significantly more impaired on measures of ADHD, personality disorder, and substance abuse and 27% had childhood ODD that had resolved. Childhood and adult ODD symptoms were significantly correlated. ODD and ADHD symptoms improved significantly with MTS ("p" less than 0.001), and the most consistently significant results were found in participants with adult ODD. Conclusion: A total of 69% met criteria for ODD as children and/or adults. Understanding how ODD interacts with ADHD to impact personality disorder, substance abuse, and treatment response has important clinical, social, and theoretical implications. (Contains 4 tables.)