Objectives. Most structural imaging studies in ADHD have focused on prefronto-striatal circuits. However, findings remained inconsistent while recent reports point to the posterior parietal cortex as an additional target for research. Moreover, although adult ADHD clinically differs from the childhood presentation little is known about the structural correlates of ADHD in adults. The aim of this study was to clarify the involvement of prefronto-striatal and posterior parietal areas in adult ADHD. Methods. Voxel-based morphometry of high resolution MRI scans was applied to analyze volumetric brain differences between 31 adult patients with ADHD and 31 control subjects. Results. The volume of prefrontal, striatal and parietal gray matter was normal. ADHD patients displayed a significant reduction of gray matter volume bilaterally in the early visual cortex (P < 0.04). Conclusions. The unexpected finding of visual cortex abnormalities might be related to impairments in early-stage, 'subexecutive" attentional mechanisms. The results support the notion that executive dysfunction may not be the dominant neurobiological characteristic of ADHD at least in adult patients. The visual cortex deserves more consideration as a potentially important site of dysfunction in adult and possibly childhood ADHD.