Proteus syndrome (PS) is a congenital disorder manifesting with severe deformities, the salient features being gigantism and vascular tumors. The disorder is poorly understood, and there has been much discrepancy in the terminology regarding the vascular tumors in PS. The purpose of this study was to elucidate the histogenesis of these tumors by correlating microscopic observations with immunohistologic information. The value of immunoperoxidase studies in the pathologic evaluation of PS was also assessed. Fourteen formalin-fixed, paraffin-embedded tissue specimens obtained from vascular tumors of six children with PS were stained with Ulex europaeus agglutinin I (UEA-I) lectin and the following immunohistochemical reagents: anti-factor VIII-related antigen (FVIII-RAg) and anti-CD34. The tumors showed varied proportions of vascular, lipomatous, and fibrous tissue components consistent with vascular hamartomas. The predominant vascular channels of the tumors were morphologically consistent with lymphatic vessels. Immunostaining of the endothelium of these vessels was most consistently positive with UEA-I lectin. Although a color reaction product was present in small vessels and some larger blood vessels, anti-CD34 immunostaining spared the lumens of lymphatic channels. In addition, a striking population of dendritic spindle cells was noted with the anti-CD34 but was unnoticed with the other reagents. We concluded that the vascular tumors of PS are primarily lymphatic hamartomas. The spindle cells noted with anti-CD34 immunostaining may relate to angiogenesis and need further delineation.