Objective: Psychological treatments for pediatric chronic pain are moderately effective. However, there have been few studies of the psychological processes associated with treatment response. This study examines the effects of Acceptance and Commitment Therapy (ACT) treatment on a severely disabled group of adolescents with chronic pain, examining relationships between outcome and acceptance. Methods: 98 adolescents with pain, mean age 15.6 years, underwent an uncontrolled trial of 3-week residential ACT treatment. Results: Adolescents improved in self-reported functioning and objective physical performance at 3-month follow-up. They were less anxious and catastrophic, attended school more regularly, and used health care facilities less often. Most positive treatment outcomes were associated with improvements in acceptance. Conclusions: An intensive ACT-based pain rehabilitation course was an effective treatment for disabled adolescents with chronic pain. Its results were theoretically consistent - improvements were associated with changes in acceptance and were achieved without pain control or cognitive restructuring techniques.