Background: Although a number of functional imaging studies are in agreement in suggesting orbitofrontal and subcortical hyperfunction in the pathophysiology of obsessive–compulsive disorder (OCD), the structural findings have been contradictory.Aims: To investigate grey matter abnormalities in patients with OCD by employing a novel voxel-based analysis of magnetic resonance images.Method: Statistical parametric mapping was utilised to compare segmented grey matter images from 25 patients with OCD with those from 25 matched controls.Results: Increased regional grey matter density was found in multiple cortical areas, including the left orbitofrontal cortex, and in subcortical areas, including the thalamus. On the other hand, regions of reduction were confined to posterior parts of the brain, such as the left cuneus and the left cerebellum.Conclusions: Increased grey matter density of frontal–subcortical circuits, consonant with the hypermetabolic findings from functional imaging studies, seems to exist in patients with OCD, and cerebellar dysfunction may be involved in the pathophysiology of OCD.