Objective : This study was designed to examine the volumetric abnormality of orbitofrontal cortex (OFC) and its change after 2 years of pharmacotherapy in obsessive compulsive disorder (OCD) patients. Method : Volumetric magnetic resonance imaging studies were conducted in 15 OCD patients and 13 normal volunteers. For 2 years, all patients took at least one serotonin reuptake inhibitor and atypical antipsychotics were used as an augmentation therapy in most patients. The follow-up MRI studies were conducted after the pharmacotherapy and OFC volumes were measured by the manual region of interest method. Results : Bilateral OFC volumes of 15 OCD patients were significantly greater than those of the normal volunteers before the treatment. After 2 years of the treatment, significant decrease was observed in bilateral OFC volumes of OCD patients to the extent that left OFC volume of OCD patients was not different from that of the normal volunteers. Conclusion : This finding suggests that OFC is directly related to the pathophysiology of obsessive compulsive disorder.