Competing hypotheses for explaining the role of anxiety in the relation between attention-deficit/hyperactivity disorder (ADHD) symptoms and childhood aggression were evaluated. Two studies tested whether anxiety exacerbated, attenuated, or had no effect on the relation between ADHD and aggression subtypes among psychiatrically hospitalized children. In Study 1 (N=99), children who scored above clinical cut-off levels for anxiety only, ADHD only, and co-occurring ADHD and anxiety were compared on aggression subtypes (i.e., reactive, proactive, overt, and relational aggression). In Study 2, the moderating role of anxiety on the relation between ADHD and aggression subtypes was examined with a larger sample (N=265) and with continuous variables. No support was found for either the attenuation or exacerbation hypothesis, and results remained consistent when separately examining hyperactivity/impulsivity and inattention symptoms of ADHD. Although ADHD symptoms were significantly associated with all aggression subtypes, this association did not remain when including symptoms of oppositional defiant disorder. © Springer Science+Business Media, LLC 2011.