Epidemiological evidence indicates that metabolic syndrome (MS) is a strong predisposing condition for atherosclerosis. Elevation of plasma low-density lipoprotein (LDL) cholesterol(LDL-C) concentration is the most important risk factor for atherosclerosis; however, LDL-C elevation is not a criterion for metabolic syndrome, raising the question of LDL's role in the syndrome's association with atherosclerosis. L5, a highly electronegative and mildly oxidized LDL subfraction that we recently isolated from hypercholesterolemic human plasma, may provide a key to answering this question. In cultured vascular endothelial cells (EC), L5 inhibits proliferation and induces apoptosis and monocyte-EC adhesion. In our preliminary studies, L5 could also be detected in patients with MS without elevated LDL-C. Because other LDL subfractions were harmless to EC, the presence of MS-L5 prompted us to hypothesize that the atherogenic role of LDL is not solely determined by plasma LDL-C concentration, but more importantly, by its composition. The proposed study is designed to test this hypothesis. The first question we will address is what lipid factor determines the prevalence of L5 in MS.