Early intervention in psychosis might be associated with better outcomes. However, intervention in the pre-psychotic phase has been questioned as, using current criteria, only 20-50% of individuals classified as prodromal develop a psychotic disorder within a 1-2 years period. Treatment agents investigated in the pre-psychotic phase of schizophrenia and other psychotic disorders should, therefore, not have major side effects. This proposal investigates omega-3 fatty acids (1.2 gramm per day eicosapentaenoic acid/docosahexaenoic acid;EPA/DHA) as a beneficial and possible preventive therapeutic agent in young people at ultra high-risk for developing a psychotic disorder.