Obstructed shunt vessels were studied immunohistochemically to clarify the mechanism of shunt obstruction in hemodialysis patients. The subjects were 12 hemodialysis patients with shunt obstruction, and 8 patients newly started on hemodialysis were used as the controls. Cryosections of shunt tissue were prepared and stained for thrombomodulin as well as transforming growth factor-β<sub>1</sub> using the enzyme antibody method. In the obstructed shunt group, the intima was significantly thicker than in the control group. In addition, staining of the intima for thrombomodulin was decreased in the obstructed shunt group when compared with the controls. Staining for transforming growth factor-β<sub>1</sub> was related to intimal thickening and cell proliferation. These results indicate that release of thrombomodulin occurs with vascular endothelial cell damage and that transforming growth factor-β<sub>1</sub> may be involved in intimal hypertrophic change and shunt obstruction.