Atherosclerosis-related diseases pose major concerns in the present day. Plasma lipoproteins have been shown closely related to the development of atherosclerosis and attention directed to qualitative alteration of lipoproteins by oxidative modification as a possible cause of atherogenesis. Oxidatively modified LDL, which is capable of inducing foam cell formation from macrophages, the adhesion of monocytes to endothelial cells, proliferation of smooth muscle cells, and other factors, may trigger atherosclerotic lesion formation. Oxidatively modified LDL has been shown present in very small amount in human circulating plasma and plasma oxidized LDL may be useful as a marker of atherosclerotic disease. The most recent information on in vivo oxidized LDL is reviewed.