This work provides a systematic review of all published randomised, controlled clinical trials (RCT) investigating the effects of n-3 PUFA intake on the prevention and treatment of non-neurodegenerative neuropsychiatric disorders. Five databases (PubMed, EMBASE, LILACS, CINAHL and The Cochrane Database) were searched for RCT in this area published up to April 2011. The selected studies all involved human participants and included a comparison group. Thirty eight studies were identified, which examined the influence of n-3 PUFA supplementation on the prevention/treatment of depression (non-perinatal) (n 23), perinatal depression (n 6) and attention deficit hyperactivity disorder (ADHD) (n 9). Great heterogeneity was noticed in terms of study design, the doses of n-3 PUFA administered, and study duration. Some benefit was noted with respect to the treatment of hyperactivity and depression in over half the examined studies, although the evidence was not conclusive. For any firm conclusions to be drawn, further studies will be needed that take into account the initial n-3 PUFA status of the subjects. Excessive n-3 PUFA intakes might be associated with a greater risk of peroxidation events and therefore neuropsychiatric deterioration. Indeed, some studies only recorded benefits when lower doses were administered. It is therefore important that the dose required to achieve any potential benefit be determined.