PURPOSE is a randomized double-blind placebo-controlled study. Main objective is to assess the effectivity of omega-3 fatty acid treatment in the prevention of psychosis. The primary outcome measure is the rate of transition to psychosis as determined through CAARMS. Subjects in the age range of 13-20 years with a higher chance of developing psychosis, as determined by the CAARMS, are treated for 6 months with omega-3 fatty acids or placebo. This study in conducted at 18 sites in 9 countries.