Endothelin is a novel potent vasoconstrictor peptide produced mainly by endothelial cells. Thrombomodulin is a high-affinity thrombin receptor on vascular endothelial cells that plays an important role as a natural anticoagulant. In this study, we measured plasma levels of endothelin and thrombomodulin in patients with systemic sclerosis or Raynaud's disease. Plasma levels of endothelin and the ratio of thrombomodulin to creatinine were significantly increased in patients with systemic sclerosis compared with normal controls, and there was a positive correlation between these two indicators (r = 0.615, P = 0.004). Moreover, plasma levels of endothelin were significantly higher in patients with diffuse systemic sclerosis than in patients with limited systemic sclerosis. In contrast, plasma levels of endothelin in patients with Raynaud's disease were not significantly increased. These results suggest that increased plasma levels of endothelin and thrombomodulin may reflect microvascular damage in systemic sclerosis.