OBJECTIVES: Adolescents with juvenile idiopathic arthritis have demonstrated substantial disagreement with their proxy's assessment of their disability, pain, and well-being. Our objective was to describe the clinical and psychological factors associated with discordance. STUDY DESIGN: This analysis included 204 proxy-adolescent (median age, 13 years) dyads that completed a Childhood Health Assessment Questionnaire for disability with 100-mm visual analogue scales for pain and well-being. Depressive symptoms in adolescents were measured by the Mood and Feelings Questionnaire and in proxies the General Health Questionnaire. Disagreement was assessed using Bland-Altman plots. Associations with discordance were identified using logistic regression analyses. RESULTS: There was higher agreement for disability (84%) than for pain (71%) and well-being (66%). Regression analyses found no association between age, sex, or disease duration and disagreement. However, relationships between disease activity and disagreement in outcomes were identified. Independent associations were found between increasing Mood and Feelings Questionnaire scores and disagreement in pain and well-being. CONCLUSIONS: Proxy and adolescent reports of pain and well-being are more likely to disagree in those with severe disease. Adolescents who report depressive symptoms are also more likely to disagree with their proxy. The reasons for these are multifactorial, and considerations of both reports are important when assessing outcomes in juvenile idiopathic arthritis.