Since our previous association studies between ADHD clinical phenotypes and these two functional polymorphisms consistently showed the low transcriptional activity alleles were preferentially transmitted to the inattentive subtype of ADHD in boys, and because these polymorphisms are all involved in the probable etiology of externalizing behaviors, the goal of the present study is to test the hypothesis that COMT Val158Met and MAOA-uVNTR jointly contribute to the phenotype of ADHD-IA with comorbid oppositional defiant disorder (ODD) in a clinical sample of Chinese male subjects. Here, COMT is linked to attention deficit-hyperactivity disorder.