Occlusive lesions in the arterial endothelium are often caused by formation of intimal hyperplasia and fibrinoid necrosis. The objective of this study was to investigate the association between antiendothelial cell antibodies (AECAs) and the development of coronary artery disease (CAD) and peripheral artery disease (PAD). In this study, 94 patients with CAD or PAD and 94 healthy volunteers serving as control subjects were examined. Frozen sections of human umbilical vein endothelial cells and primate smooth muscle cells were used to detect the presence of AECAs, which were found in 52 of 94 patients (55%) and in 15 of 94 controls (16%) (P < .001). Endothelial structure tissue damage is a major factor in arterial diseases. In the present study, a statistically significant relationship was found between AECAs and the development of CAD and PAD. The presence of AECAs has been identified as a risk factor for these diseases. According to this study, AECAs are reliable prognosticators for the development of CAD and PAD. Further studies with large numbers of serum samples are under way.