The incidence of obsessive-compulsive disorder (OCD) in patients with Gilles de la Tourette's syndrome (GTS) was assessed with a specially designed questionnaire. The Inventory was administered to patients with OCD, patients with GTS, and normal controls. Fifty-one percent of the patients with GTS had significantly elevated Inventory scores. The frequent occurrence of OCD in GTS suggests that the two disorders may share common neurobiologic mechanisms.