One in five U.S. youngsters has a mental disorder, but we know little about the effects of these disorders on child outcomes. We examine U.S. and Canadian children with symptoms of Attention Deficit Hyperactivity Disorder (ADHD), the most common child mental health problem. Our innovations include the use of nationally representative samples of children, the use of questions administered to all children rather than focusing only on diagnosed cases, and the use of sibling fixed effects to control for omitted variables. We find large negative effects on test scores and schooling attainment suggesting that mental health conditions are a more important determinant of average outcomes than physical health conditions.