OBJECTIVE: To analyze patterns of stress, mood, disease symptoms, and activity reduction in children with polyarticular arthritis, using a prospective daily diary method. METHODS: For a 2-month period, 51 children with polyarticular arthritis (mean age 12.4 years, 65% female) completed daily diaries that included measures of symptoms of pain, stiffness, and fatigue, as well as stress, mood, and activity reduction. Functional status and disease activity were assessed at the initial and followup evaluations with use of the Childhood Health Assessment Questionnaire, physician global assessment, joint count, and laboratory testing. RESULTS: Children reported having pain, stiffness, and fatigue on >70% of days, with significant variability in symptom levels. Results revealed significant same-day relationships between stress, mood, and disease symptoms, after controlling for covariates. Specifically, daily fluctuations in both stress and mood were predictive of increased pain, stiffness, and fatigue. Increases in daily stress, mood, and disease symptoms were also significantly related to decreased participation in social activities on a day-to-day basis. Only mood and stiffness were predictors of a cutback in school attendance. CONCLUSION: Stress and mood are important predictors of daily disease symptoms in children with polyarticular arthritis. Moreover, daily fluctuations in stress, mood, and disease symptoms are predictive of aspects of daily function, including participation in school and social activities. Thus, health care providers should solicit daily symptom reports when making decisions regarding clinical management. Nonpharmacologic interventions such as cognitive-behavioral therapy and stress management may be useful adjuvant therapy when treating the disease symptoms of children with polyarticular arthritis.