The present study investigated the antifatigue effects of Panax ginseng C.A. Meyer in 90 subjects (21 men and 69 women) with idiopathic chronic fatigue (ICF) in a randomised, double-blind, placebo-controlled and parallel designed trial. A bespoke 20% ethanol extract of P. ginseng (1 g or 2 g day^-1) or a placebo was administered to each group for 4 weeks, and then fatigue severity was monitored using a self-rating numeric scale (NRS) and a visual analogue scale (VAS) as a primary endpoint. Serum levels of reactive oxygen species (ROS), malondialdehyde (MDA), total glutathione (GSH) contents and glutathione reductase (GSH-Rd) activity were determined. After 4-week, P. ginseng administration decreased the total NRS score, but they were not statistically significant compared with placebo (P>0.05). Mental NRS score was significantly improved by P. ginseng administrations as 20.4 ± 5.0 to 15.1 ± 6.5 [95% CI 2.3 ~ 8.2] for 1 g and 20.7 ± 6.3 to 13.8 ± 6.2 [95% CI -0.1 ~ 4.2] for 2 g compared with placebo 20.9 ± 4.5 to 18.8 ± 2.9 [95% CI 4.1 ~ 9.9, P<0.01]. Only 2 g P. ginseng significantly reduced the VAS score from 7.3 ± 1.3 to 4.4 ± 1.8 [95% CI 0.7∼1.8] compared with the placebo 7.1 ± 1.0 to 5.8 ± 1.3 [95% CI 2.2 ~ 3.7, P<0.01]. ROS and MDA levels were lowered by P. ginseng compared to placebo. P. ginseng 1 g increased GSH concentration and GSH-Rd activity. Our results provide the first evidence of the antifatigue effects of P. ginseng in patients with ICF, and we submit that these changes in antioxidant properties contribute in part to its mechanism. Clinical Research Information Service (CRIS) KCT0000048.