BACKGROUND: Atypical antipsychotics may have efficacy as augmentation therapy in treatment resistant depression (TRD) but evidence is limited. METHODS: An open label study of quetiapine augmentation in 24 patients (mean age: 46.3 years) with a DSM-IV major depressive episode resistant to at least 2 trials of antidepressant medication, and currently taking a monoamine reuptake inhibitor. An 8-week treatment phase was followed by an 18-week extension in patients who showed clinical benefit. RESULTS: Eighteen patients (75%) completed the 8-week treatment phase with seven patients (29%) being responders on the Montgomery Asberg Depression Rating Scale and 13 (54%) on the CGI-I. Fewer patients responded if they had previously received olanzapine in the current episode but this was not statistically significant (0% v 37%, p=0.27). Of the eleven patients entering the extension phase, 3 patients (27%) experienced a significant worsening of mood. The most common adverse events were sedation (54%), dry mouth (38%) and dizziness (29%). Significant weight gain was found in 40% of patients treated for 26 weeks. Average quetiapine doses were 245 mg at 8 weeks and 346 mg at 26 weeks. CONCLUSIONS: Quetiapine may be a helpful adjunctive agent for some patients with TRD but placebo-controlled trials are needed to establish its place in management. LIMITATIONS: The trial was open-label and the numbers were small.