This is a clinicopathologic study of five patients with intravascular papillary endothelial hyperplasia involving the orbit and ocular adnexa. The median age of the patients was 45 years. Three lesions were located in the orbit, one in the eyelid, and one in the eyebrow. Four lesions were located within a distended vein and one involved an artery. The walls of the vessel corresponded to the "capsule" of the circumscribed mass. The lumen disclosed different stages of organization of a thrombus with prominent endothelial cell proliferation centered around cores of collagenous tissue and fibrin. Electron microscopy of one lesion displayed spindle-shaped cells with features of endothelial cells forming a polarized basement membrane with numerous micropinocytotic vesicles on their plasmalemma. Some cells showed ultrastructural features of pericytes. The lesion, which may be confused with angiosarcoma, represents an unusual exuberant proliferation of vascular endothelium as a cellular response to the organization of a thrombus.