Is obsessive-compulsive disorder (OCD) a discrete disorder? Three hundred thirty-four individuals with OCD were interviewed using the Structured Clinical Interview for DSM (SCID). Results demonstrate that OCD is highly comorbid with other neuropsychiatric disorders, with 92% of OCD study participants receiving one or more additional Axis I DSM diagnoses. Among these additional diagnoses, lifetime mood disorders (81%) and anxiety disorders (53%) were the most prevalent. With the exception of substance-related disorders and specific phobias, all disorders assessed were found in considerably higher frequency than in the general population, indicating that OCD is associated with highly complex comorbidity. These data have implications for genetic studies of OCD and disorders related to OCD, as well as for specific psychotherapeutic and psychopharmacologic interventions.