To examine the pathogenesis of intravascular papillary endothelial hyperplasia (IPEH), a relatively uncommon benign, non-neoplastic vascular lesion, clinicopathological and immunohistochemical studies were performed. Paraffin-embedded tissue specimens of 78 vascular lesions were examined histologically, and 9 cases of IPEH were investigated immunohistochemically using antibodies against CD34, vimentin, factor VIII antigen, α-smooth muscle actin (α-SMA), podoplanin, CD105, and ki-67 antigen. A thrombus or ulcer was found near the sites of all IPEH specimens. Histologic examination revealed papillary proliferated endothelial cells located toward the lumen of enlarged blood vessels. Immunohistochemistry showed that CD34, α-SMA, and factor VIII antigen were positive in lining endothelial cells. Vimentin was positive in the mesenchymal components. Immunohistochemical staining for podoplanin and CD105 was partially positive. Labeling index was 4.7 to 9.2 in ki-67-positive cases. IPEH is believed to result from reactive proliferation of blood endothelial cells that is caused by an abnormal process of organization in thrombosed blood vessels. The pathogenesis of IPEH might be related to inflammation or mechanical stimulus such as irritation.