Seventy-eight cases of spindle cell hemangioendothelioma (SCH) were studied to reevaluate its pathogenesis and determine its long-term behavior. Most of the original findings were confirmed by this study. The tumor occurred at all ages (8-78 years; median, 32 years; mean, 34 years). Males and females were equally affected. The tumor developed as a superficially located mass of the distal extremities (upper, 32 cases; lower, 30 cases). Four patients (5%) also had Maffucci's syndrome. The lesions were circumscribed red-brown masses occasionally containing phleboliths that consisted of cavernous blood spaces alternating with cellular areas consisting of collapsed vascular spaces separated by spindled fibroblastic cells. Often the endothelium lining the collapsed blood spaces appeared epithelioid with cytoplasmic vacuolization. The spindled fibroblastic cells lacked significant atypia and had at most a low level of mitotic activity. As a significant departure from what was originally reported, more than half of these cases (58%) were partially or completely intravascular. The vein containing the tumor often had an irregularly attenuated wall with small intimal herniations and intimal papillae traversing the lumen. Similar intimal changes in adjacent vessels suggest that SCH grows as a multifocal or contiguous process within vessels. Follow-up information was obtained in 40 cases, ranging from 1 month to 40 years (mean, 5.4 years). Despite conservative excisions in most patients (simple excision, 83%; wide local excision, 13%; amputation, 2%), prognosis was excellent. Fifty-eight percent experienced recurrences, but no patient developed metastasis and no patient died of the direct effects of the tumor, although one patient with Maffucci's syndrome developed a concurrent angiosarcoma. We conclude that SCH is a primary benign vascular neoplasm or malformation similar to angiomatosis in which alterations in blood flow might explain some of the secondary features. Areas of diminished blood flow result in vascular collapse with formation of the "cellular" zones, and areas of vascular engorgement with stasis promote thrombosis and organization. Local "recurrences" probably represent contiguous spread along or multifocal involvement of a vessel. Because there is no evidence that this lesion has metastatic potential, we suggest that the lesion be designated spindle cell hemangioma for solitary lesions and spindle cell hemangiomatosis for multifocal lesions.