Five soft tissue hemangiopericytomas exhibiting the range of histologic types common to this tumor were studied with the electron microscope. Morphological correlates revealed that the better differentiated pericytes were present in the tumors or areas thereof displaying open capillaries, scant connective tissue, and oval, plump tumor cells. These findings support the concept of a pericytic histogenesis of this neoplasm. Well-differentiated pericytes were harder to find in areas with perivascular fibrosis and in tumors featuring spindle-shaped pericytes. Apparent forms of transition between pericytes and endothelial cells were common, especially in the less well-differentiated tumors. In one of these, atypical endothelial cells resembling those of endothelial sarcomas were present. These findings are suggestive of a close histogenetic relationship between hemangiopericytoma and hemangioendothelioma.