A growing body of evidence suggests that omega (ω)-3 polyunsaturated fatty acids (PUFAs) are clinically useful in patients with psychiatric disorders. In the present review, we summarize the findings of randomized, placebo-controlled clinical trials that have focused on the potential therapeutic utility of ω-3 PUFAs in patients with mental illnesses. We searched the PubMed database for placebo-controlled clinical trials using the keywords "PUFAs", "omega-3", "eicosapentaenoic acid", and "docosahexaenoic acid" in combination with the following terms: "anxiety disorders", "mood disorders", "autism", "attention-deficit hyperactivity disorder" (ADHD), "personality disorders", and "schizophrenia". The literature review indicated that personality disorders, autism, and anxiety disorders have been investigated less frequently than mood disorders, schizophrenia, and ADHD. Although no definite conclusions can be drawn on the therapeutic efficacy of ω-3 PUFAs in the majority of the psychiatric illnesses examined here, the evidence suggests that these molecules have a potential preventive role in people at extremely high risk for developing psychosis. Future studies in the field should examine ω-PUFAs turnover in neural membranes. Moreover, special attention should be paid to potential confounds, such as smoking and dietary habits.