PUFA have become the subject of growing number of studies concerning supplementation in the treatment of mental disorders and diseases in recent years. It is suggested that some mental illnesses are associated with cellular abundance of omega-3 PUFA or/and reduced dietary intake. PUFA probably affect dopaminergic and serotonergic neurotransmission so their use in attention deficit, hyperactivity disorder and depression might be effective. Researchers have explored the effect of n-3 PUFA in a range of different mental health problems from developmental disorders in childhood to depression, bipolar disorder, schizophrenia, cognitive decline and Alzheimer’s disease. The aim of this review was to assess the role of n-3 PUFA and to determine whether n-3 PUFA are likely to be efficacious in these disorders. Only randomized, placebo-controlled trials published before August 2012 were analysed. Finding of clinical trials are inconsistent and methodological differences between studies, like sample size, selection criteria, dosage of supplement and length of supplementation, the type of fatty acids causes that the role of PUFA in psychiatry is still open. It is not determined what fatty acids and in what proportions should be applied. Analysed trials suggest that monotherapy with eicosapentaenoic acid (EPA) or in combination with small doses of docosahexaenoic acid (DHA) are most effective.