Objective: Children with arthritis experience frequent pain, but the predictors of daily pain variations are largely unidentified. The goal of this study was to examine sleep quality as a predictor of pain in children with arthritis and to determine whether mood moderates this relationship. Method: In this prospective, longitudinal study children with polyarticular arthritis (n = 51, ages 8–16 years) tracked daily symptoms, including sleep quality over 2 months. Self-reported daily pain intensity, as indicated on a visual analog scale, was used as the primary outcome measure in multilevel models. Results: Poorer sleep quality was associated with higher next-day pain ratings (p < .01). Mood moderated this relationship such that as positive mood increased, the relationship between poor sleep quality and high pain weakened (p < .01). Daily pain did not predict nightly sleep quality (p > .05). Conclusions: Sleep quality is an important predictor of pain in children with arthritis. These findings add to the growing body of literature on the use of daily diaries for analyzing patterns of pain, sleep, and mood in children with chronic painful conditions.