We have previously demonstrated that human aortic endothelium exhibits morphologic heterogeneity in situ, and this heterogeneity can be reproduced in culture. In this study, we have compared prothrombotic properties of cultured endothelial cells (EC) from areas of human aorta at high risk for atherosclerosis (HP-EC) with EC from areas at low risk (LP-EC). Using paired cultures from the same donors, we have found that the expression of cell surface thrombomodulin (TM) - as measured by the ability to generate activated protein C (APC) from protein C in the presence of thrombin - is relatively reduced on HP-EC compared to LP-EC (respectively, 4.98 ± 4.43 vs. 5.83 ± 4.37 pM APC/min/cm2; p=.03, n=12). Furthermore, HP-EC more efficiently assemble the prothrombinase complex on their cellular surface, resulting in an increased rate of thrombin generation from prothrombin (9.81 ± 3.10 (HP-EC) vs. 7.96 ± 3.20 nM thrombin/min/cm2 (LP-EC); p <.03, n =7). The combination of reduced TM expression and increased prothrombinase complex assembly on HP-EC suggests a prothrombotic phenotype in these cells. These findings may be important in the pathogenesis of thrombosis associated with atherosclerotic plaques.