Major recurrent mood disorders including major depressive disorder (MDD) and bipolar disorder (BD) are associated with significant psychosocial morbidity and excess premature mortality primarily attributable to suicide and coronary heart disease. Limited efficacy and adverse side-effects associated with psychotropic medications used in the treatment of MDD and BD highlight the urgent need to develop safe and efficacious treatments or treatment adjuncts. A body of evidence now indicates that long-chain omega-3 (LCn-3) fatty acid deficiency is a feature associated with MDD and BD. The etiology of LCn-3 deficits in MDD and BD patients may be attributable to both genetic and environmental factors. Dietary LCn-3 supplementation is safe and well-tolerated with chronic administration and corrects LCn-3 deficiency in MDD and BD patients. LCn-3 supplementation has been found to augment the therapeutic efficacy of psychotropic medications in the treatment of mood symptoms and to reduce suicidality. Preliminary studies also suggest that LCn-3 supplementation is efficacious as monotherapy in the treatment and prevention of psychopathology in children and adolescents. LCn-3 supplementation is also associated with reduced risk for developing coronary heart disease. The overall cost-benefit ratio associated with LCn-3 supplementation provides a strong rationale to diagnose and treat LCn-3 deficiency in MDD and BD patients, and to prevent LCn-3 deficiency in subjects at high risk for developing these disorders.