Although the Yale-Brown Obsessive Compulsive Scale (Y-BOCS) and the Padua Inventory-Revised (PI-R) are the most widely used instruments for assessing presence and severity of symptoms in obsessive-compulsive disorder (OCD), the correlation between the two instruments is surprisingly low. The aim of the present investigation was to test two possible explanations for the discrepancy between these instruments by comparing both scales in 120 OCD patients. The tested hypotheses included: (1) differences in the way the measures are administered, i.e. observer-rated vs. self-rated; and (2) differences in the way severity is calculated, i.e. the PI-R measures severity by aggregating symptoms, while the Y-BOCS measures severity unrelated to the number of symptoms. Results indicated that neither hypothesis satisfactorily explains the differences between the measures. The investigation concludes that the Y-BOCS and PI-R measure relatively unrelated features of OCD, and the combined use of multiple measures is recommended to assess the complexity of OCD phenomena.