A series of 106 cases of hemangiopericytoma was analyzed. The neoplasms occurred principally in adults (median age, 45 years), were deep seated, and were most common in the thigh (27 cases) and the pelvic retroperitoneum (26 cases). A painless mass was the first symptom in 96 of the patients. The median size of the excised tumors was 6.5 cm. Surgical removal of the tumor was often complicated by hemorrhage because of marked dilatation of the vascular bed in the vicinity of the neoplasm, probably as the result of rapid exchange of blood from the arterial to the venous circulation within the tumor. Microscopically, benign and malignant forms could be distinguished. The latter were characterized by increased cellularity, prominent mitotic activity, and foci of necrosis or hemorrhage. Follow-up information was obtained in 93 cases. Seventy-one of the 93 patients were living (two with recurrence and four with metastasis), and 22 had died (13 as the result of recurrence or metastasis in more than two-thirds of the related causes). Recurrence preceded metastasis in more than two-thirds of the patients with evidence of metastasis. The 10 year survival rate was 70 per cent. The morphologic differences from other mesenchymal tumors showing a hemangiopericytoma-like vascular pattern are discussed, and the close resemblance of hemangiopericytoma to richly vascular forms of fibrous histiocytomas and synovial sarcoma is emphasized. Congenital or infantile hemangiopericytoma is described as a separate entity having a distinctive microscopic pattern and behavior.