Objective: To evaluate the association between the clinical scales of the child behavior checklist (CBCL) and the comorbid diagnosis of oppositional defiant disorder (ODD) in a large sample of youth with attention deficit hyperactivity disorder (ADHD). Method: The sample consisted of 101 girls and 106 boys ages 6 to 17 with ADHD. Conditional probability analysis was used to examine the correspondence between CBCL Clinical Scales with the structured-interview derived diagnosis of ODD. Results: Conditional probability analysis showed that the CBCL Aggression Scale best predicted a structured-interview derived diagnosis of ODD in boys and girls with ADHD. Conclusion: These findings suggest that the CBCL Aggression Scale could serve as a rapid and cost-effective screening instrument to help identify cases likely to meet clinical criteria for ODD in the context of ADHD. (Contains 2 tables and 2 figures.)