Application of theoretically based tasks to the study of the development of selective attention has led to intriguing new findings concerning the role of inhibitory mechanisms. This study examined inhibitory mechanisms using a countermanding task and an inhibition of return task to compare deficits in intentionally, versus reflexively, controlled inhibition of attention in children with attention deficit hyperactivity disorder. Fifty children with attention-deficit/hyperactivity disorder (ADHD) were classified into one of three subtypes: predominantly inattentive (ADHD/PI), combined (ADHD/C), and those children with ADHD/C who also met criteria for comorbid oppositional defiant disorder (ADHD/C + ODD). The groups were compared to a comparison group of children (n = 21). The countermanding task showed that the ADHD groups required more time to inhibit responses and this impairment did not differ among subtypes. With respect to reflexively controlled inhibition, compared with controls ADHD/C and ADHD/C + ODD groups showed impaired reflexive inhibition, whereas the ADHD/PI group was considerably less impaired. The findings highlight a dissociation between the two forms of inhibitory deficits among children with the inattentive subtype, and raise the possibility that the efficient operation of reflexive inhibitory mechanisms might be necessary for the development of effective intentional control of inhibition.