We investigated the immunohistochemical localization of plasminogen activator inhibitor type 1 (PAI-1) in surgical specimens of 41 human intracranial tumors. Malignant tumors (7 glioblastoma multiforme, 5 anaplastic gliomas, 4 malignant meningiomas, and 9 metastatic tumors) showed consistently stronger PAI-1 immunohistoreactivity than benign or low-grade tumors (5 low-grade gliomas, 8 benign meningiomas, and 3 Schwannomas). Strong PAI-1 positivity was confined to glomeruloid-shaped proliferative vessels seen in high-grade gliomas and metastatic tumors. Blood vessels near necrotic foci and some zones of necrosis also showed strong PAI-1 positivity. PAI-1 localizes in the vascular basement membrane and perivascular connective tissue, while endothelial cells themselves show weak positivity. None of the specimens showed PAI-1 reactivity within the tumor cells per se. Localization of PAI-1 in proliferating vessels suggests that PAI-1 may be involved in angiogenesis.