In order to understand the relationship between lipid peroxide in blood serum and various diseases supposed to be related to abnormal lipid metabolism, lipid peroxides in human patient serums were measured by fluorometry. The results obtained were folllowing: In normal human, the serum lipid peroxides showed a slight increase by age, but not much difference by sex. Lipid peroxides in blood serum of various patients with hypertention, diabetes, hepatitis, arteriosclerosis, stomach cancer were higher than those of the normal human. From these results obtained, the autors concluded that lipid peroxides in blood serum appeared to be correlated to various diseases supposed to be realted to lipid peroxidation.