Fifteen patients with DSM-III-R diagnosis of obsessive-compulsive disorder (OCD) were rated according to the Turkish version of the Yale Brown Obsessive Compulsive Scale (Y-BOCS) and the US National Institute of Mental Health Global Obsessive Compulsive Scale (NIMH-GOCS) by 7 raters independently from audiotaped interviews. Patients also completed the Maudsley Obsessive Compulsive Inventory (MOCI). Interrater reliability of Y-BOCS and NIMH-GOCS were very good as well as correlations between these two scales. The correlations of MOCI with Y-BOCS and NIMH-GOCS were not significant. We found Y-BOCS and NIMH-GOCS to be reliable and valid instruments in assessing the severity of OCD. These findings suggest that MOCI may not be a suitable instrument for assessing the severity of OCD.