The purpose of this study was to determine whether attention-deficit/hyperactivity disorder (AD/HD) is uniquely related to impairments in two aspects of executive functioning: (1) response inhibition: and (2) response re-engagement. AD/HD (n = 10), disruptive (n = 11), anxious (n = 11) and normal children (n = 21) were compared on the change task. Children were in the age range of 8 12 years. The psychopathological groups were recruited from special educational services. Parent, teacher and child questionnaires were used to select children with pervasive disorders. Controls attended normal classes and scored low on all questionnaires. Compared with normal children, both AD/HD and disruptive children showed poor response inhibition, but only AD/HD children exhibited a deficit in the underlying inhibitory process. Some evidence was found for enhanced response inhibition in anxious children. Both AD/HD and disruptive children demonstrated higher variability in the speed of the response re-engagement process and were less accurate. The results suggest that AD/HD involves a more pervasive impairment in cognitive functioning, rather than a deficit restricted to the powers of response inhibition.