BACKGROUND: Previous studies in predominantly bipolar patients have suggested that gabapentin may be useful in treating mood disorders. This report describes its efficacy and tolerability as an adjunctive agent in treatment-resistant depression. METHODS: A chart review was conducted on 27 outpatients presenting with a depressive disorder in whom gabapentin was added to ongoing treatment with a conventional antidepressant to which patients had not responded after at least 6 weeks. The majority of patients had either prominent anxiety or a history of soft bipolar features, but patients with bipolar I disorder were excluded. Clinical state and adverse effects were assessed retrospectively at each visit. RESULTS: Mean gabapentin trial duration was 15.2+/-7.8 weeks, with a mean final dose of 904+/-445 mg/day (range, 300-1800 mg/day). Clinician-rated measures of clinical state improved significantly from baseline to endpoint. Overall, 37.0% (n=10) of patients were responders at endpoint; another 18.5% (n=5) manifested a transient response not sustained to endpoint. Gabapentin was well tolerated; the most common adverse effects were fatigue, sedation, dizziness, and gastrointestinal symptoms. LIMITATIONS: Treatment was uncontrolled and efficacy assessments were retrospective. CONCLUSION: These findings suggest that gabapentin may be of adjunctive benefit in the management of treatment-resistant depression.