Objectives: Chronic Fatigue Syndrome and Fibromyalgia (CFS/FMS) are debilitating syndromes affecting ~2- 4% of the population. Although they are heterogeneous conditions associated with many triggers, they appear to have the common pathology of being associated with impaired energy metabolism. As D-ribose has been shown to increase cellular energy synthesis, and was shown to significantly improve clinical outcomes in CFS/FMS in an earlier study, we hypothesized that giving D-ribose would improve function in CFS/FMS patients.