Recently, much attentions have been paid on lipid peroxides in human body, mainly because of their involvement in pathogenesis of many degenerative diseases such as atherosclerosis and the aging. In the present lecture, a micro-method for the assay of serum lipid peroxide level, which was devised by the lecturer, will be introduced first. Then, the intervention of lipid peroxides in pathogenesis of some diseases will be mentioned. The relation to the vascular disorders will be described from both experimental and clinical observations. The role of lipid peroxides in provoking cataract will be proposed. It will be emphasized that the pathological increase of lipid peroxides in any tissue or organ results in their leakage to the blood serum, which in turn injures an intact tissue or organ.