Oxidized low-density lipoprotein (ox-LDL) is considered to play a key role in the genesis of inflammatory processes in atherosclerotic lesions. It has also been shown that LDL isolated from patients with diabetes mellitus (DM) has an enhanced susceptibility to oxidation. Recently, a sandwich ELISA method for measurement of plasma ox-LDL levels has been developed. To elucidate the role of ox-LDL in plaque instability in coronary artery disease, we measured the plasma ox-LDL levels in patients with acute myocardial infarction (AMI), unstable angina pectoris (UAP), and stable angina pectoris (SAP), and moreover assessed whether a relationship is present between plasma ox-LDL levels and DM. We also measured the plasma ox-LDL level in a patient who died of AMI, thus enabling us to study the presence of ox-LDL and CD 36, which is one of the ox-LDL receptors, in the culprit lesion. Plasma ox-LDL levels were measured in 210 patients (AMI: 70, UAP: 70, SAP: 70), and in 55 control subjects. Plasma ox-LDL levels in AMI patients were significantly higher than in UAP patients (P<.0001), SAP patients (P<.0001), or controls (P<.0001). In the UAP group, plasma ox-LDL levels in patients with DM were significantly higher than those without DM (P<.005). The autopsied patient who died of AMI revealed an increased plasma level of ox-LDL, and immunohistochemically, the culprit coronary lesion contained abundant macrophage-derived foam cells, showing distinct positivity for ox-LDL and CD 36. These results strongly suggest an important role for ox-LDL in the genesis of plaque instability in human coronary atherosclerotic lesions.