Papillary endothelial hyperplasia is a peculiar benign intravascular process that bears a remarkable resemblance to a hemangiosarcoma. In 44 cases of this lesion studied from the files of the Armed Forces Institute of Pathology, the process manifested as a small tumor-like lesion that occurred most frequently in the subcutis of the fingers (14 cases), the head and neck region (ten), and the trunk (seven). Microscopically, the tuft-like or papillary proliferation of endothelial cells was nearly always intimately associated with a thrombus and seemed to represent a peculiar variant of an organizing process. Features that aided in recognition and differential diagnosis from a hemangio-sarcoma included the intraluminal location of the lesion, the absence of tissue necrosis, and the intimate association of the proliferated tuft-like structures with thrombotic material. Follow-up information obtained in 31 cases indicated a benign clinical course despite the sarcoma-like microscopic appearance of this condition.