Objective: Impaired peer relationships have long been recognized as one of the major functional problems of children with ADHD, but no specific guidelines on clinical levels of impairment in this domain exist. Method: This study used Receiver Operating Characteristics methodology to determine what aspects of peer functioning best discriminate between children with ADHD and their classmates. Optimal cutoffs indicative of clinical levels of impairment associated with ADHD diagnosis were determined for all variables. The participants were 165 children with AD/HD who were part of the Multimodal Treatment Study of Children With ADHD and their 1,298 classmates. Results: Variables that best discriminated between children with ADHD and their classmates included peer rejection and negative imbalance between given and received liking ratings (i.e., children with ADHD liked others more than they were liked). Conclusion: Peer rejection and negative imbalance show most promise for identifying clinically significant levels of peer relationship impairment in children with ADHD. © 2009 Sage Publications.