Lipid peroxide in human skin surface lipids was investigated. The correlation between the amount of lipid peroxide and the total amount of lipids in skin surface lipids was determined by statistical analysis. Furthermore, the correlation between lipid peroxide and squalene in skin surface lipids was also proved. These results suggest that the major source of lipid peroxide in skin surface is squalene, and that lipid peroxidation is enhanced by external factors such as ultraviolet light irradiation.