Attention-deficit/hyperactivity disorder (ADHD) is the most commonly diagnosed mental health disorder among U.S. children. Diagnosis can bring positives, like proper treatment, extra testing time, and social support, but may also trigger negatives, like stigmatization. Although rates of diagnosis are high across socioeconomic status (SES) groups, the balance of positive and negative consequences of diagnosis may differ by SES. In high-SES communities, mental health diagnoses are less stigmatized and parents have greater ability to connect children to support resources, suggesting greater positive effects of diagnosis for high-SES children. Alternatively, the greater academic pressure present in high-SES communities may amplify the negative effects of mental health stigma, suggesting larger negative diagnostic effects. Using the Early Childhood Longitudinal Study–Kindergarten Cohort of 1998–1999, I found that diagnosed and medicated high-SES but not low-SES children exhibit significantly poorer future self-competence and teacher-rated school behaviors than undiagnosed matches. Findings suggest that diagnosis may not always be a net positive.