Obsessive–compulsive disorder (OCD) is characterized by recurrent, intrusive, and disturbing thoughts, as well as by repetitive stereotypic behaviour. Insight into the senseless nature of the symptoms is generally preserved. Patients try, albeit usually unsuccessfully, to suppress the obsessive thoughts and compulsive behaviours. Acting out the stereotypic behaviours reduces the anxiety generated by the obsessions and compulsions (APA, 2000). In 60% of patients OCD develops before the age of 25 yr, and onset of disease can already occur in childhood (Flament et al., 1990). Familial loading is higher in early-onset OCD, indicating that genetic factors may be of greater importance in OCD with early onset (Pauls et al., 1995).