The mechanisms underlying the debilitating CFS are poorly understood. Autonomic symptoms are key in diagnosing CFS, yet the specific role of the sympathetic nervous system (SNS) in CFS remains elusive. Dysfunction of the SNS manifests in the very symptoms often displayed in CFS, yet to date, its activity has not been directly assessed in CFS. For the first time, we will use the innovative and specialised technique of microneurography to definitively define and directly measure sympathetic activity in CFS. This will deepen our understanding of the role of the SNS in CFS symptoms.