Objective: Longitudinal magnetic resonance imaging (MRI) studies have shown reduced cortical thickness (CT) in individuals with ADHD, but this abnormality disappears with age, suggesting developmental delay. However, cross-sectional MRI studies have shown reduced CT, suggesting abnormal development. The aim of this study was to compare whole-brain CT in male and female children, adolescents, and adults with ADHD with whole-brain CT in matched control participants. Method: MRI scans were performed on ADHD and control participants. Results: CT data revealed differences in right hemisphere (RH) only. Reduced CT was observed predominantly in the frontoparietal region. However, increased CT was observed predominantly in the occipital lobe. The CT differences were correlated with severity of ADHD. Analysis of sex differences revealed that location, number, and magnitude of CT differences were different between males and females in each age group. Conclusion: These data support the hypothesis that anatomical abnormalities in ADHD represent abnormal development rather than developmental delay. (J. of Att. Dis. 2012; XX(X) 1-XX)