The aim of this study was to identify differences in brain activation between Attention Deficit Hyperactivity Disorder (ADHD) patients and typically developing pre-adolescents and adolescents while performing a working memory task. Within this important cognitive domain, visuo-spatial working memory was specifically selected for study, being a fundamental cognitive impairment in the disorder. Functional magnetic resonance imaging (fMRI) was used to measure brain activity in children and adolescents between 12 and 18 years of age while they performed a visuo-spatial working memory task. During performance of the task the ADHD group showed lower activation than the typically developing children in 4 specific brain regions. Between-group differences in activation were observed in the right and left superior frontal gyrus, right superior temporal gyrus, and cuneus. The results of the present study support the hypothesis of mainly cortical dysfunctions in ADHD.