Although the adaptive role of positive affect (PA) in childhood internalizing disorders is well-established, less is known about PA in children with AttentionDeficit/Hyperactivity Disorder (ADHD). In this cross-sectional study, we examined associations between child-reported PA and parent-reported emotional and behavioral problems in youth with ADHD using multiple hierarchical regression analyses. Participants included 848 ethnically diverse youth (253 youth with ADHD, 595 with other or no diagnoses; age range 6.12-19.65) referred for mental health assessments. Study measures included the Child Behavior Checklist (CBCL) and the Positive and Negative Affect Scale for Children (PANAS-C). Results indicated that higher levels of child-reported PA were associated with greater parental reports of total and internalizing problems among youth with ADHD but not in other clinicreferred youth. Specifically, child self-reports of PA were positively associated with parental reports of anxious/depressed and somatic problems in youth with ADHD. In addition, associations between child-reported PA and parent-reported problems differed by type of ADHD comorbidity. Findings suggest that at higher levels, PA in youth with ADHD might exert a paradoxical effect and be a subtle risk factor for or an indicator of parent-reported internalizing problems among youth with ADHD only and youth with ADHD and a comorbid externalizing disorder but not for youth with ADHD and a comorbid internalizing disorder or youth without ADHD. Discrepancies between child-reported PA and parent-reported problems may influence treatment planning and treatment response. Further clinical and theoretical implications for child therapists and parents are discussed.