VASCULAR tumors have been of particular interest to surgeons for their unusual clinical and pathologic characteristics and because of the technical ingenuities frequently required for their removal. Stout's description of a previously unreported tumor of vascular origin, which he named hemangiopericytoma,1has caused increased interest in the subject.2 This tumor is composed of numerous capillary blood vessels around which are grouped masses of perivascular cells. Stout believes that these perivascular tumor cells derive from Zimmermann's pericytes,3those highly contractile, smooth, muscle-like cells which lie outside the capillaries. In distinction from hemangioendothelioma in which the tumor cells lie within the capillaries, the cells in hemangiopericytoma lie completely outside the vascular spaces. Their extraluminal situation can be demonstrated best by Laidlaw's silver stain, which brings the connective tissue fibers of the capillaries into sharp relief. The tumor occurs as a firm encapsulated mass, at times with obvious blood vessel