We measured plasma endothelin-1 (ET-1) levels in patients with Takayasu's arteritis to investigate its relationship with the disease activity. We also measured plasma thrombomodulin (TM) levels as a marker of endothelial injury. A significant positive correlation was evident between plasma ET-1 levels and erythrocyte sedimentation rates (ESRs) in patients with Takayasu's arteritis (r = 0.53, p < 0.005). In contrast, plasma levels of TM did not show a significant correlation with ESRs (r = -0.13) ET-1, secreted in this way during the active stages of Takayasu's arteritis, may cause constriction and proliferation of vascular smooth muscle cells and, as a result, progression of luminal narrowing of the affected vessels in this disorder.