The unique objectives of the current investigation were: (a) to assess the fit of a multiinformant 2-factor measurement model of friendship quality in a clinical sample of children with attention-deficit/hyperactivity disorder (ADHD); and (b) to use a multiple indicators multiple causes approach to evaluate whether comorbid externalizing and internalizing disorders incrementally predict levels of positive and negative friendship quality. Our sample included 165 target children diagnosed with ADHD (33% girls; aged 6-11 years). Target children, their parents, their friends, and the parents of their friends independently completed a self-report measure of friendship quality about the reciprocated friendship between the target child and the friend. Results indicated that a multiinformant 2-factor measurement model with correlated positive friendship quality and negative friendship quality had good fit. The friendships of children with ADHD and a comorbid externalizing disorder were characterized by less positive friendship quality and more negative friendship quality than the friendships of children with ADHD and no externalizing disorder after controlling for the presence of a comorbid internalizing disorder. However, the presence of a comorbid internalizing disorder did not predict positive or negative friendship quality. These findings suggest that soliciting reports from parents in addition to children and friends, and measuring comorbid externalizing disorders, may be valuable evidence-based strategies when assessing friendship quality in ADHD populations. (PsycInfo Database Record (c) 2020 APA, all rights reserved).