Objective: This study aimed to investigate the cortical thickness in areas of the brain that are hypothesized to be involved in response inhibition and error-monitoring behaviors. The authors hypothesized that children with ADHD would have a thinner prefrontal cortex (PFC) and anterior cingulate cortex (ACC) than healthy children. Method: In all, 25 ADHD and 25 healthy control male children (5-12 years) underwent magnetic resonance imaging. Results: The authors found thinner right superior frontal gyrus in ADHD patients compared with controls (t = 2.01, df = 45, p = .049). The older children with ADHD drove this effect when participants were further subdivided into a younger and older age group (older participants: p = .004; younger participants: p = ns). Conclusion: These findings have implications for the developmental trajectory of the frontal lobe in ADHD. (J. of Att. Dis. 2012; XX(X) 1-XX)