Attention-deficit/hyperactivity disorder (ADHD) is the most common behavioral disorder among U.S. children. ADHD diagnoses have risen among children with both severe and mild behavioral problems, partly in response to mounting academic pressure. This study examines the consequences of ADHD diagnosis. Diagnosis can bring beneficial pharmacological treatment and social supports, but it can also trigger negative social and psychological processes, as suggested by labeling theory. For children with mild behavioral problems, diagnosis may trigger awareness of being ‘‘different’’ for the first time, for example through negative teacher/peer effects. By matching diagnosed and otherwise comparable undiagnosed children from the Early Childhood Longitudinal Study-Kindergarten Cohort of 1998–1999, I find that medication has positive effects for diagnosed children with severe prediagnosis behavioral problems, yielding comparable future teacher-rated school behaviors as undiagnosed matches. However, diagnosed and medicated children with mild prediagnosis behavioral problems exhibit poorer future teacher-rated social and academic behaviors than their undiagnosed matches, consistent with labeling theory.