The immunocytochemical study of thrombomodulin (TM), a newly recognized anticoagulant endothelial surface protein, was performed with a surgical specimen of a superficial temporal artery (STA) obtained from a 29-year-old woman with familial moyamoya disease. The staining of TM showed positive immunoreactivity in smaller vessels in the surrounding connective tissue of the specimen, whereas negative in STA. Immunoelectronmicroscopically the luminal plasma membrane of endothelial cells was positive for TM. These staining pattern was the same as that in controls. She concurrently suffered from von Willebrand disease type I, and she had two cerebral hemorrhagic attacks. A quantitative defect of the von Willebrand factor in the endothelium was demonstrated immunocytochemically.