The sarcomatous component of gliosarcomas is thought by many to originate from the vascular proliferation seen in glioblastoma multiforme and has, therefore, been assumed to be endothelial. Immunohistochemical staining of four gliosarcomas has led us to an alternate theory. Pathologically all four tumors were composed of at least two cell types; the first had a stellate, glial appearance and the second was either spindled or polygonal in shape. Polygonal cells were associated with glomeruloid vascular structures in some areas. Both components of each neoplasm were cytologically malignant. Glial fibrillary acidic protein and S-100 antibodies stained most of the glial-appearing cells and some of the spindled cells, but not the polygonal cells. Muscle specific alpha-actin and smooth muscle specific alpha-actin antibodies stained only the malignant spindled and polygonal cells and normal vascular smooth muscle. Ulex europaeus agglutinin I and anti-factor VIII/related antigen antibody stained only cells lining vascular lumina. The staining results suggest that the malignant mesenchymal component of these tumors is of smooth muscle origin. Having demonstrated elsewhere that glomeruloid vascular structures of glioblastoma multiforme contain smooth muscle cells, we propose here that gliosarcomas can represent one end of the spectrum of glioma-induced vascular smooth muscle proliferation.