Objective: This cross-sectional study sought to confirm the presence and regional profile of previously reported changes in laminar cortical thickness in children and adolescents with attention-deficit/hyperactivity disorder (ADHD) compared with typically developing control subjects. Method: High-resolution magnetic resonance images were obtained from 22 (19 male and 3 female subjects; mean age 11.7 years) children and adolescents with ADHD and 22 age- and sex-matched control subjects (mean age 11.7 years). Brain tissue volumes were estimated for each subject. Cortical pattern matching methods were used to sample measures of laminar thickness at high spatial frequency across homologous regions of the cortex. Volume and thickness measures were compared across diagnostic groups with and without controlling for general intelligence. False discovery rate correction confirmed regional results. Results: The subjects with ADHD exhibited significant reductions in overall brain volume, gray matter volume, and mean cortical thickness compared with the controls, whereas white matter volumes were significantly increased in ADHD. Highly significant cortical thinning (false discovery rate-corrected p less than 0.0006) was observed over large areas of the frontal, temporal, parietal, and occipital association cortices and aspects of motor cortex but not within the primary sensory regions. Conclusions: Cortical thickness reductions present a robust neuroanatomical marker for child and adolescent ADHD. Observations of widespread cortical thinning expand on earlier cross-sectional findings and provide further evidence to support that the neurobiological underpinnings of ADHD extend beyond prefrontal and subcortical circuits. (Contains 1 table and 2 figures.)