Fibromyalgia (FM) is characterized by widespread pain and co-morbid symptoms such as fatigue and depression. For FM, medical treatments alone appear insufficient. Recent meta-analyses point to the utility of cognitive behaviour therapy (CBT), but effects are moderate. Within the continuous development of CBT, the empirical support for acceptance and commitment therapy (ACT) has increased rapidly. ACT focuses on improving functioning by increasing the patient's ability to act in accordance with personal values also in the presence of pain and distress (i.e., psychological flexibility). However, no study has yet explored the utility of ACT in FM. To evaluate the efficacy of ACT for FM and the role of psychological inflexibility as a mediator of improvement. In this randomized controlled trial, ACT was evaluated in comparison to a waiting list control condition. Forty women diagnosed with FM participated in the study. Assessments were made pre- and post-treatment and at 3 months of follow-up. The ACT intervention consisted of 12 weekly group sessions. Significant differences in favour of ACT were seen in pain-related functioning, FM impact, mental health-related quality of life, self-efficacy, depression, anxiety and psychological inflexibility. Changes in psychological inflexibility during the course of treatment were found to mediate pre- to follow-up improvements in outcome variables. The results correspond with previous studies on ACT for chronic pain and suggest the utility of ACT for FM as well as the role of psychological inflexibility as a mediator of improvement. © 2012 European Federation of International Association for the Study of Pain Chapters.