This study is an extension of Colledge and Blair’s (2001) investigation on the inter-relationship between core features of attention deficit/hyperactivity disorder and conduct disorder. The current study extended the earlier work using methodological refinements in sample selection, psychiatric diagnostic assessment, and instruments used to measure inattention, hyperactivity, and psychopathy. The primary outcome was that the impulsiveness-conduct problem component of psychopathy was specifically associated with inattention and hyperactivity components of attention deficit/hyperactivity disorder. Impulsivity may be an underlying mechanism common to both disorders and may represent a selective target for treatment.