This controlled parent-targeted drug prevention intervention followed two cohorts of students to determine the effect of encouraging parents to intervene in risk factors for adolescent tobacco and alcohol use. Suggestions included limiting association with substance-using peers and asking parents to limit adolescent access to alcohol. Although student participation and retention in the study was good parent participation was poor. Parents were unlikely to know whether their children had substance-using friends and there was no impact on adolescent substance use. However, parent monitoring of children's whereabouts, maintaining high rapport and a respectful parent-child relationship did protect against adolescent substance use. Future parent-targeted prevention programs should target protective factors, rather than trying to control risk factors.