Four patients with hemangiopericytoma were examined with gray-scale sonography. Three of these patients were also evaluated by computed tomography (CT). The sonographic findings included well-defined, predominately cystic masses in two patients, a mixed solid and cystic mass in one, and a solid mass with shadowing in another. One hemangiopericytoma changed in sonographic appearance from cystic to solid following radiation. CT identified well-circumscribed masses in all three cases, but added little beyond what was seen on plain radiographs. Sonography may define the limits of a hemangiopericytoma, give information about the internal structure, and offer a way of following response to therapy. However, the sonographic features are varied and the often cystic nature of these tumors may be misleading.