The aim of this study was to evaluate the benefit of oral ubiquinol-10 supplementation in CFS patients using an open-label study and a randomized, double-blinded, placebo-controlled (RCT) study. Twenty patients with CFS were randomly enrolled in an 8-week open-label oral ubiquinol-10 (150 mg ubiquinol-10/day) study. The patients and the attending physicians were not blinded to the supplementation. Forty-three patients with CFS were randomly assigned to receive either ubiquinol-10 (150 mg/day) or placebo every day for 12 weeks. The patients and the attending physicians were blinded to the supplementation, and a total of 31 patients (N = 17 in the ubiquinol group and 14 in the placebo group) completed the study. The beneficial effects of ubiquinol-10 were observed in the open-label study we conducted prior to the RCT. The RCT results suggest that supplementation with ubiquinol-10 for 12 weeks is effective for improving several CFS symptoms. © 2016 BioFactors, 42(4):431-440, 2016.