Fibromyalgia (FM) is a common disorder characterized by generalized pain and fatigue as well as many other somatic and psychiatric symptoms. FM leads to large suffering, high disability, and large societal costs. The most evaluated treatment is traditional cognitive behavior therapy (CBT), which has only shown modest effects in FM. We have previously developed and evaluated exposure-based CBT for patients with FM. Exposure involves to voluntarily and systematically approach avoided situations and stimuli associated with pain and discomfort. Participants (N=140) were randomized to 10 weeks of internet-delivered exposure therapy (iExp) or waitlist control. Results from the trial showed that iExp rendered large effects on FM symptoms, pain, fatigue, disability and pain-related worry, and that iExp is highly cost-effective compared to wait-list. Thus, the results are promising, but a clear limitation is that iExp has not yet been evaluated against an active control condition. We are now applying for funds to conduct a large-scale randomized controlled trial comparing iExp to traditional CBT. In addition to studying clinical effects we will also evaluate cost-effectiveness and investigate underlying factors that drive the treatment effect, in order to further enhance the treatment. Positive results from this project would imply that iExp can soon be implemented within regular healthcare and distributed to patients with FM nationally.