Objective: To examine precursors of adolescent conduct disorder (CD) in children with attention-deficit/hyperactivity disorder (ADHD), investigating the significance of childhood oppositional defiant disorder (ODD) and ADHD. Method: A total of 151 children with ADHD recruited from child psychiatric and pediatric clinics were assessed through standardized diagnostic interviews at ages 6 to 13 years and in adolescence 5 years later. Using multiple regression analysis, we assessed baseline ODD diagnosis and ODD, CD, and ADHD symptom scores as clinical predictors of adolescent CD diagnosis and symptom scores. Results: Childhood ODD (diagnosis and severity) was significantly associated with adolescent CD (diagnosis and severity), independent of childhood ADHD severity and childhood CD. Children with a diagnosis of ODD were almost three times more likely to develop CD in adolescence (odds ratio = 2.79, 95% CI 1.16-6.70, p = 0.02). Childhood ADHD severity predicted adolescent CD scores but not diagnosis of CD (although there was a trend toward association). The presence of at least one CD symptom in childhood predicted adolescent CD severity. Conclusions: ODD is a significant precursor of adolescent CD in children with ADHD independent of ADHD severity. Considering the negative prognosis of ADHD with comorbid CD, it is imperative that clinicians pay specific attention to the presence of childhood ODD behaviors. (Contains 3 tables and 1 figure.)