BACKGROUND: Trials have demonstrated benefits of long-chain omega-3 essential fatty acid (n-3 EFA) supplementation in a variety of psychiatric disorders. AIMS: To assess the efficacy of n-3 EFAs in improving psychological well-being in patients with recurrent self-harm. METHOD: Patients (n=49) presenting after an act of repeated self-harm were randomised to receive 1.2 g eicosapentaenoic acid plus 0.9 g decosahexaenoic acid (n=22) or placebo (n=27) for 12 weeks in addition to standard psychiatric care. Six psychological domains were measured at baseline and end point. RESULTS: At 12 weeks, the n-3 EFA group had significantly greater improvements in scores for depression, suicidality and daily stresses. Scores for impulsivity, aggression and hostility did not differ. CONCLUSIONS: Supplementation achieved substantial reductions in surrogate markers of suicidal behaviour and improvements in well-being. Larger studies are warranted to determine if insufficient dietary intake of n-3 EFAs is a reversible risk factor for self-harm.