Background <p>The prevalence of depressive and anxiety disorders dramatically increases as children enter adolescence. It is critical to identify etiological factors that can assist in the identification of children most at risk. Event-related potentials are one measure that has demonstrated promise and feasible application in children and adolescents. In particular, the P300 has been extensively employed to examine cognitive system deviations associated with depressive and anxiety disorders. However, this work has primarily focused on adults, and there have been limited prospective investigations, making it unclear whether the P300 can prospectively predict the development of later depressive and anxiety disorders during critical developmental periods, such as adolescence.</p> Methods <p>The present sample included 272 9-year-old children with no history of psychopathology who completed the doors task while continuous electroencephalography was recorded to measure the choice- and feedback-locked P300s. Participants completed follow-up diagnostic interviews through age 15 to determine onset of later depressive and anxiety disorders.</p> Results <p>A smaller choice-locked, but not feedback-locked, P300 in childhood predicted an increased likelihood of developing first-onset depression by mid-adolescence. Neither P300 predicted the development of anxiety disorders.</p> Conclusions <p>The present study indicates a blunted choice-locked P300 indexes risk for depressive disorders in adolescence. The choice-locked P300 might be a valuable neural measure for further understanding pathways unique to increasing depression in adolescence.</p>