Endosialin (tumor endothelial marker 1) is expressed preferentially by tumor endothelial cells but not by normal endothelium. Its protein domain architecture is homologous to that of CD93 and thrombomodulin (CD141), suggesting a similar function in mediating cell-cell interactions. The aim of this study was to investigate the expression pattern of endosialin in human brain tumors in a bid to decipher its contribution to tumor angiogenesis. We generated an antibody specifically recognizing human endosialin and used it to study endosialin expression in 30 human brain tumor specimens by immunoblotting and immunohistochemistry. Twenty of 30 tumors expressed endosialin in a heterogeneous manner. The largest proportion of endosialin-expressing tumors was found in highly invasive glioblastoma multiforme, anaplastic astrocytomas, and metastatic carcinomas. Endosialin was localized to the endothelium of small and large vessels strongly stained for CD31 and was also expressed by Thy-1-positive fibroblast-like cells close to the meninges and alpha-smooth muscle actin-positive cells in some vessels. Endosialin colocalized with thrombomodulin, suggesting the proteins may have complementary functions in tumor progression.