Anxiety and mood disorders (AMD) may be more common among adolescents than previously thought, and epidemiological research would benefit from an easily-administered measure of AMD. We assessed the ability of the GHQ12 to estimate the prevalence of AMD in a representative sample of Ontario adolescents. Data were based on self-administered questionnaires from 3,311 students in grades 7 through 12 derived from the 2007 Ontario Student Drug Use and Health Survey. The GHQ12 showed good ability to identify probable AMD (AUC = 0.80). A threshold value of 5 appeared optimal, and based on this threshold value, the estimated prevalence of AMD in Ontario students in grades 7-12 may be 19%. Observations that individuals with a self-reported history of family disruption, substance use, gambling problems, bullying victimization, and who reported seeking professional help for a mental health problem were significantly more likely to experience probable AMD provided additional validity.