OBJECTIVE: Studies have reported that countries with high rates of fish oil consumption have low rates of depressive disorder. The authors studied a specific omega-3 fatty acid, the ethyl ester of eicosapentaenoic acid (E-EPA), as an adjunct to treatment for depressive episodes occurring in patients with recurrent unipolar depressive disorder who were receiving maintenance antidepressant therapy. METHOD: Twenty patients with a current diagnosis of major depressive disorder participated in a 4-week, parallel-group, double-blind addition of either placebo or E-EPA to ongoing antidepressant therapy. Seventeen of the patients were women, and three were men. RESULTS: Highly significant benefits of the addition of the omega-3 fatty acid compared with placebo were found by week 3 of treatment. CONCLUSIONS: It is not possible to distinguish whether E-EPA augments antidepressant action in the manner of lithium or has independent antidepressant properties of its own.