Lipid peroxidation products have received a great deal of attention for the practical application to clinical use as bio-markers. F2-isoprostanes from arachidonates and neuroprostanes from docosahexanoates have been proposed as bio-markers. Although these markers are formed by a free radical-mediated oxidation, the yields from the parent lipids are minimal. In contrast, hydroperoxyoctadecadienoates from linoleates and oxysterols from cholesterols are yielded by much simpler mechanisms from more abundant parent lipids in vivo. We proposed the method in which both free and ester forms of hydroperoxides and ketones as well as hydroxides of linoleic acid and cholesterol are measured as total hydroxyoctadecadienoic acid (tHODE) and 7-hydroxycholesterol (t7-OHCh), respectively. The concentrations of tHODE and t7-OHCh in physiological samples were much higher than that of 8-iso-prostagrandin F_<2α>. In addition to this advantage, hydrogendonor activity of antioxidants in vivo could be determined by the stereoisomeric-ratio of HODE.