It has been claimed that excessively positive self-perceptions of competence are a key risk factor for concurrent and subsequent impairments in youth with attention-deficit/ hyperactivity disorder (ADHD). We examined whether girls with ADHD demonstrate positive illusory self-perceptions in scholastic competence, social acceptance, and behavioral conduct domains. We then tested, across a five-year longitudinal span, whether (a) such self-perceptions versus (b) the constituent informant ratings or test scores were more strongly predictive of adolescent impairment and positive adjustment. Participants included an ethnically diverse sample of 140 girls with ADHD and 88 comparison girls, aged 6-12 at baseline (M = 9.0, SD = 1.7). Girls with ADHD rated themselves as more positive than indicated by external ratings, but these self-reports were still in a negative direction (comparison girls rated themselves as less positive than these indicators). ADHD subtypes were not related to discrepancy scores. Higher rates of depression symptoms were associated with attenuated discrepancy scores. Crucially, measures of actual competence were more strongly associated with adolescent impairment and positive adjustment than were "illusory" self-perceptions for girls with ADHD. Our findings challenge the view that, at least in girls with ADHD, overly positive and "illusory" appraisals of competence are strongly associated with future impairment and adjustment. The key psychometric point is that, in difference or discrepancy scores, the individual components of such scores should be separately examined.