Major depression and dysthymia are frequent, debilitating, and chronic disorders, whose highest rate of initial onset is during the late adolescent years. The effectiveness of a program designed to prevent an initial episode of major depression or dysthymia among adolescents was investigated. Participants were 171 fourteen-year-old "at risk" Icelandic adolescents who were randomly assigned to a prevention program or a treatment-as-usual assessment only control group. They were identified as "at risk" by reporting the presence of depressive symptoms or a negative attributional style. The program was based on a developmental psychosocial model of enhancement of resilience to factors associated with the occurrence of mood disorders. The results indicated that the prevention program resulted in a significantly lower rate of major depression and dysthymia than did the control group. The study demonstrated that school personnel in the school setting can implement such prevention programs.