Given that 5-HT3 receptors are indirect inhibitors of cortico-mesolimbic DA release, the 5-HT3 receptor antagonist ondansetron augmentation might potentially have efficacy in the treatment of resistant Obsessive Comulsive Disorder (OCD) patients on combined SRIs and antipsychotics.

METHOD: Fourteen patients with a DSM-IV diagnosis of treatment resistant OCD, under stable treatment with SSRI's and neuroleptic augmentation will enter an 12-week single blind trial of ondansetron initiated at a dose of 0.25 mg twice daily for 6 weeks, that will be titrated to 0.5 mg twice daily for 6 weeks.

Inclusion Criteria:

1. adults aged 18 to 55
2. a Yale-Brown Obsessive-Compulsive Scale (Y-BOCS) score of ≥ 20 after ≥ 12 weeks of treatment with an established effective dose of an Selective Serotonin Reuptake Inhibitors or clomipramine and after ≥ 10 weeks of augmentation treatment with antipsychotics (risperidone at least 2 mg/day quetiapine at least 150 mg/day; olanzapine at least 5mg/day; haloperidol titrated at least 10 mg/day; aripiprazole at least 10 mg/day)

Exclusion Criteria:

1. a history of alcohol or substance abuse
2. current severe depressive symptoms, bipolar disorder, panic disorder, schizophrenia, or other psychiatric conditions
3. heart disease, arrhythmia, liver problems, including cirrhosis, seizures, glaucoma or serious medical disease
4. hoarding as only Obsessive Compulsive symptom
5. women of childbearing potential not using a medically acceptable contraceptive method.