This study aimed to determine the efficacy and tolerability of adding quetiapine to a serotonin reuptake inhibitor in treatment-resistant obsessive-compulsive disorder (OCD). Twenty-one adult treatment-resistant OCD patients were randomized to 16 weeks of augmentation with either quetiapine (n = 11) or placebo (n = 10). Patients with significant comorbidities, including tic-spectrum disorders, were not included. The treatment was well tolerated, with only one premature dropout in each treatment-group. The primary analysis showed that individuals in the quetiapine-treated group showed a 14% mean improvement in baseline Yale-Brown Obsessive-Compulsive Scale scores at study endpoint compared with a 6% improvement in those treated with placebo, but this difference did not reach statistical significance (F<1). Three patients treated with quetiapine met criteria for clinical response, compared to one patient who was treated with placebo. Larger studies are needed to explore the efficacy of second generation antipsychotics, such as quetiapine, when used as adjunct treatment in resistant OCD.