['The aim of this study was to examine the prospective relationship between a history of attention-deficit/hyperactivity disorder (ADHD) assessed in mid-adolescence and the onset of major depressive disorder (MDD) through early adulthood in a large school-based sample. A secondary aim was to examine whether this relationship was robust after accounting for comorbid psychopathology and psychosocial impairment.', 'One thousand five hundred seven participants from the Oregon Adolescent Depression Project completed rating scales in adolescence and structured diagnostic interviews up to four times from adolescence to age 30.', 'Adolescents with a lifetime history of ADHD were at significantly higher risk of MDD through early adulthood relative to those with no history of ADHD. ADHD remained a significant predictor of MDD after controlling for gender, lifetime history of other psychiatric disorders in adolescence, social and academic impairment in adolescence, stress and coping in adolescence, and new onset of other psychiatric disorders through early adulthood (hazard ratio, 1.81; 95% confidence interval, 1.04, 3.06). Additional significant, robust predictors of MDD included female gender, a lifetime history of an anxiety disorder, and poor coping skills in mid-adolescence, as well as the onset of anxiety, oppositional defiant disorder, and substance-use disorder after mid-adolescence.'] Does attention-deficit/hyperactivity disorder in adolescence predict onset of major depressive disorder through early adulthood?