Oxidative stress is thought to play a central role in low-density lipoprotein (LDL) oxidation, and the oxidative modification of LDL is thought to be a key factor during early atherogenesis. Therefore, the prevention of LDL oxidation by antioxidants might arrest the progression of atherosclerosis. This study was conducted to determine the effect of glabridin, a natural polyphenolic isoflavone antioxidant isolated from licorice root, on LDL oxidation. The determination of the extent of LDL oxidation was accomplished by measuring the formation of thiobarbituric acid reactive substances (TBARS). The oxidative stress level was assessed using a FORM system/CR 3000 instrument. After the oral administration of a glycyrrhizin-free licorice-root methanol extract (containing glabridin 60 mg) to healthy subjects for 6 months, the subjects' oxidative stress level as well as plasma LDL oxidation decreased by 20%. We conclude that the dietary consumption of glabridin can partially protect LDL from oxidation.