Although parents and teachers are valid informants in the assessment of childhood attention-deficit/hyperactivity disorder (ADHD), there is relatively little systematic research on how these ratings should be optimally combined. We compared four methods of ADHD assessment to determine how well they identified impaired children: (1) parent only, (2) teacher only, (3) parent or teacher (‘or rule’), and (4) parent and teacher (‘and rule’). We obtained parent and teacher ratings of ADHD from the Disruptive Behavior Disorder Rating Scale on 232 5- to 10-year-old children (69% male; 47% Caucasian) with (n = 121) and without (n = 111) ADHD. We used receiver operating characteristic curves (ROC) and seemingly unrelated regression analyses (SUR) to evaluate how accurately each method identified categorically- and dimensionally-defined measures of functional impairment. Parent ratings of ADHD optimally identified globally impaired children based on categorical and dimensional measures. However, teacher ratings of ADHD most accurately identified children who were negatively regarded by peers using categorical, but not dimensional, measures. No ADHD assessment method effectively identified children with academic difficulties. Although multiple informants are valuable in the assessment of ADHD, no single method was consistently superior in identifying impaired children across domains. We consider alternative assessment strategies in ADHD as well as other potential factors that may contribute to modest agreement among informants.