This paper evaluates the internal consistency reliability and concurrent validity of the assessment of Diagnostic and Statistical Manual of Mental Disorders (DSM-IV) attention deficit hyperactivity disorder (ADHD) in the adolescent version of the World Health Organization (WHO) Composite International Diagnostic Interview Version 3.0 (CIDI). The CIDI is a lay-administered diagnostic interview that was carried out in conjunction with the US National Comorbidity Survey Adolescent Supplement, a US nationally representative survey of 10,148 adolescents and their parents. Internal consistency reliability was evaluated using factor and item response theory analyses. Concurrent validity was evaluated against diagnoses based on blinded clinician-administered interviews. Inattention and hyperactivity-impulsivity items loaded on separate but correlated factors, with hyperactivity and impulsivity items forming a single factor in parent reports but separate factors in youth reports. We were able to differentiate hyperactivity and impulsivity factors for parents as well by eliminating a subset who endorsed zero ADHD items from the factor analysis. Although concurrent validity was relatively weak, decomposition showed that this was due to low validity of adolescent reports. A modified CIDI diagnosis based exclusively on parent reports generated a diagnosis that had good concordance with clinical diagnoses [area under the curve (AUC) = 0.78]. Implications for assessing ADHD using the CIDI and the effect of different informants on measurement are discussed.