© The Royal College of Psychiatrists 2017. Background There is accumulating evidence for the role of fronto-striatal and associated circuits in obsessive-compulsive disorder (OCD) but limited and conflicting data on alterations in cortical thickness. Aims To investigate alterations in cortical thickness and subcortical volume in OCD. Method In total, 412 patients with OCD and 368 healthy adults underwent magnetic resonance imaging scans. Betweengroup analysis of covariance of cortical thickness and subcortical volumes was performed and regression analyses undertaken. Results Significantly decreased cortical thickness was found in the OCD group compared with controls in the superior and inferior frontal, precentral, posterior cingulate, middle temporal, inferior parietal and precuneus gyri. There was also a group6age interaction in the parietal cortex, with increased thinning with age in the OCD group relative to controls. Conclusions Our findings are partially consistent with earlier work, suggesting that group differences in grey matter volume and cortical thickness could relate to the same underlying pathology of OCD. They partially support a frontostriatal model of OCD, but also suggest that limbic, temporal and parietal regions play a role in the pathophysiology of the disorder. The group6age interaction effects may be the result of altered neuroplasticity.