Oxidized low-density lipoprotein (OxLDL) has previously been thought to promote atherogenesis through foam cell formation. However, the actual nature of OxLDL present in vivo remained obscure until recently. We have produced a monoclonal antibody, DLH3, which specifically binds to OxLDL but not to native LDL. The presence of OxLDL in the LDL fraction of human plasma was demonstrated by introducing a sandwich ELISA procedure using DLH3 together with an anti-apoB antibody. Furthermore, OxLDL levels appeared to increase in certain pathological conditions including acute myocardial infarction and carotid artery atherosclerosis. Accumulation of OxLDL in atherosclerotic lesions has also been demonstrated by immunohistochemical and biochemical studies using the DLH3 antibody. This antibody recognizes oxidized phosphatidylcholines (OxPC) generated during oxidative modification of LDL, and OxPC-apoB adducts formed in OxLDL are the presumed antigens. Measuring OxLDL in plasma would be a useful diagnostic tool for cardiovascular diseases. However, there still remain some major questions related to OxLDL, the answers to which are crucial for understanding the mechanisms of atherogenesis.