Cavernous angiomas (CAs) and arteriovenous malformations (AVMs) sometimes show growth or de novo appearance. Proliferative capacity of the endothelium is evident in such malformations. Intermediate filament nestin is a newly identified marker for proliferative endothelium, which was originally detected in the neuroepithelial stem cells of the embryonal central nervous system. Immunohistochemical analysis of nestin was evaluated as a marker for proliferative capacity of endothelial cells by comparison with proliferating cell nuclear antigen (PCNA). Sixteen of 27 CAs and 13 of 20 AVMs were positive for nestin. Likewise, 12 of 27 CAs and 10 of 20 AVMs were positive for PCNA. Nestin staining was stronger in the typical malformative vessels of CAs than in AVMs, in both the endothelial cells and the interstitial cells. Newly formed endothelial cells expressed nestin strongly in the reactive tissues including organizing or encapsulated hematomas. These results suggest that neovascularization occurs in the process of repeated bleeding and remodeling of hematomas.