Objective To use receiver-operating characteristics analysis to identify multilevel diagnostic likelihood ratios and provide a framework for the diagnosis of attention-deficit/hyperactivity disorder (ADHD) in children (5–10 years old) and adolescents (11–18 years old) in an outpatient setting. Method Caregiver, teacher, and youth reports from the Achenbach System of Empirically Based Assessment (ASEBA) were obtained for 299 children and 321 adolescents with multiple imputation of missing data. The reference standard was diagnosis of ADHD based on case history and a semistructured diagnostic interview masked to the ASEBA measurements. Results In children, caregiver-reported Attention Problems (area under the curve [AUC] = 0.74) outperformed all other subscales of the caregiver and teacher measures (AUCs ≤ 0.72). In the older sample, caregiver- and teacher-reported Attention Problems (caregiver AUC = 0.73; teacher AUC = 0.61) were best at identifying ADHD. Inclusion of caregiver and teacher reports significantly (p < .001 for all comparisons) increased prediction of ADHD diagnosis, whereas youth self-report did not. Conclusion Caregiver-reported Attention Problems were more useful than teacher-reports and self-report in identifying ADHD. Combining caregiver and teacher reports improved identification. Multilevel likelihood ratios are provided to facilitate routine clinical use.