Although several magnetic resonance imaging (MRI) studies have been conducted in adults with obsessive-compulsive disorder (OCD), few studies have used voxel-based morphometry to examine brain structure, especially in psychotropic drug-naive pediatric patients. MRI examinations of 37 psychotropic drug-naive pediatric OCD patients and 26 age- and sex-matched healthy volunteers were acquired on a 1.5 T MRI system, normalized to a customized template, and segmented with optimized voxel-based morphometry. Pediatric OCD patients had significantly more gray matter in regions predicted to differ a priori between groups, including the right and left putamen and orbital frontal cortex. Among patients, more gray matter in the left putamen and right lateral orbital frontal cortex correlated significantly with greater OCD symptom severity, but not with anxiety or depression. Manual region-of-interest measurements confirmed more gray matter in the orbital frontal cortex and putamen in patients compared to healthy volunteers. More anterior cingulate gray matter was evident among patients compared to healthy volunteers with regional volumetry but not with voxel-based morphometry. Regions of significantly less gray matter in OCD were confined to the occipital cortex and were not predicted a priori. Our results suggest that OCD is characterized by more gray matter in brain regions comprising cortical-striatal-thalamic-cortical circuits. These findings are consistent with functional neuroimaging studies reporting hypermetabolism and increased regional cerebral blood flow in striatal, anterior cingulate, and orbital frontal regions among OCD patients while in a resting state.