This study assessed children’s overestimations of self-competence (positively biased self-perceptions or positive bias [PB]) relative to parent/teacher ratings of children’s competence in predicting children’s adjustment in a new setting. Eighty-five children (13 boys and 11 girls with Attention-Deficit/Hyperactivity Disorder [ADHD]; 30 boys and 31 girls who were typically developing [TD]), ages 6.8 to 9.8 years (M = 8.13; SD = 0.82), attended a 2-week summer day camp grouped into same-age, same-sex classrooms with previously unacquainted peers and counselors. Prior to camp, PB was assessed by creating standardized discrepancy scores between children’s self-ratings relative to parent or teacher ratings of the children’s social and behavioral competence. The relative ability of these discrepancy scores to predict peer preference and oppositionality at camp in relation to parent or teacher ratings alone was evaluated. For children with ADHD, both discrepancy scores and informant ratings of competence were uniquely predictive of peer preference and oppositionality assessed during camp. For TD children, only informant ratings of competence were predictive of outcomes at camp. These results suggest that PB may be a unique predictor of maladjustment within a novel environment for children with ADHD, but not TD children.