• The change in Yale Brown obsessive compulsive scale (Y-BOCS) from baseline to week 10 and the number of responders are the primary efficacy parameters.
• Criteria for response will be a 25% or greater change from baseline on the Y-BOCS and a final CGI rating of "much improved or "very much improved".

• The onset of response to treatment, using the time to a sustained response as criterion
• Side effect profiles, Quality of life, Cognitive functioning

Quetiapine (Seroquel ®), an atypical antipsychotic registered for use in schizophrenia, which has a very low propensity of extrapyramidal and endocrine side-effects, has also been studied as an adjunct in OCD. In an open trial, ten patients with OCD who had not responded to at least three previous treatments with a SRI at maximum dose and duration were assigned to receive quetiapine in addition to a SRI for 8 weeks. Given the efficacy of quetiapine in treatment resistant patients, and given its rapid onset of action (4-6 weeks), it is postulated that the combination of a low dose atypical antipsychotic and a standard dosage of an SRI as a treatment for patients with OCD might increase the number of responders as well as the effect size.

PLEASE NOTE: Seroquel SR and Seroquel XR refer to the same formulation. The SR designation was changed to XR after consultation with FDA.

Inclusion Criteria:

• All patients meet the DSM IV criteria for obsessive-compulsive disorder
• Y-BOCS score > 16 if obsessions and compulsions
• Y-BOCS score > 10 if only obsessions
• Y-BOCS score > 10 if only compulsions
• Male and female, aged between 18-70 years
• Female patients of childbearing potential must have a negative pregnancy test and use a reliable method of contraception.
• Written informed consent

Exclusion Criteria:

• Presence of any of the following DSM IV conditions; major depression (with a HDRS>15, [17 item]), bipolar disorder, schizophrenia or any other psychotic condition, tic disorder, substance related disorder during the past 6 months, epilepsy, or any structural CNS disorder or stroke within the last year.
• Evidence of clinically significant and unstable cardiovascular, gastro-intestinal, pulmonary, renal, hepatic, endocrine or haematological disorders, glaucoma, myocardial infarction within the past year, or micturition abnormalities
• Patients at risk for suicide
• Multiple serious drug allergies or known allergy for the trial compounds
• Use of antipsychotics during 6 months before the screening visit
• Use of any other psychotropic drug during 6 months before the screening visit
• Cognitive and behavioural treatment 3 months prior to the screening visit
• Any known contra-indication against citalopram or quetiapine