Obsessive-compulsive disorder (OCD) is a relatively common, often chronic and disabling disorder with high rates of partial and/or absent response to standard, recommended treatments, such as selective serotonin reuptake inhibitors (SSRIs) and psychotherapy. This article presents the cases of four patients suffering from OCD and comorbid mood or anxiety disorders, who were treated with SSRIs at adequate doses for at least 12 weeks, showing a partial response. Quetiapine treatment was added to SSRIs at a dose of 25 mg/day and titrated up to 200 mg/day. Patients were followed up for 6 months. After 12 weeks, all the patients were classified as "much improved" on the Clinical Global Impression-Improvement scale and showed a Yale-Brown Obsessive-Compulsive Scale score reduction > or =35%. After 6 months of follow-up, all the patients maintained the same level of improvement. Although quetiapine augmentation to SSRIs has shown mixed results in published controlled trials in the acute treatment (12 weeks) of patients with treatment-resistant OCD, this case series indicates that patients who benefit from this pharmacologic regimen in the acute phase tend to maintain such an improvement. Larger follow-up studies are warranted to confirm our findings.