Oxidative modification of low density lipoproteins and tissue lipids has been proposed to be involved in the pathogenesis of atherosclerosis. We examined human atherosclerotic lesions of various stages from fifteen victims of acute heart failure and detected substantial amounts of oxygenated fatty acids in the tissue ester lipids. The degree of lipid oxygenation correlated with the stage of advancement of the lesion. More than 85% of the oxygenated fatty acids were localized in the cholesterol esters, whereas phospholipids contained only small amounts. Structure elucidation of the oxygenation products indicated a nonspecific product pattern of various isomers of keto- and hydroxy-octadecadienoic acid. The data presented suggest an involvement of lipid peroxidation in the pathogenesis of atherosclerosis and indicate that the majority of the oxygenation products are formed via nonspecific, non-enzymatic reactions possibly initiated by the action of a 15-lipoxygenase.