Irritability is ubiquitous in child and adolescent psychopathology. This study aimed to determine if the Affective Reactivity Index (ARI), a measure of irritability, could be used to screen for psychopathology in adolescents. The clinical sample comprised 31 adolescents with a DSM-IV diagnosis. The control sample was 31 gender and age matched adolescents recruited through schools. Both samples completed a test battery that included the Affective Reactivity Index. The clinical participants reported significantly higher levels of irritability than the control sample by both self- and parent-report. Using ROC analysis a cut off value of 4 on the self-report ARI was found to be optimal for indicating psychopathology; with a specificity of 77.4% and a sensitivity of 77.4%, the area under the curve was 0.86. This paper provides evidence to suggest that irritability may be used as a general predictor of psychopathology in adolescents.