To advance the neurobiology and treatment of obsessive-compulsive disorder (OCD) by focusing on Tourette's Syndrome (TS)-spectrum OCD as a possible homogeneous form of OCD, and investigating the relevance of intact 5-hydroxytryptamine (5-HT) function to the mechanism of anti-OC drug action. The validity of TS-spectrum OCD as a distinct subtype is assessed using a detailed clinical, family, drug treatment response profile in adult OCD patients.