A case of rapidly fatal, high-grade leiomyosarcoma of the upper extremity is described. The pleomorphic appearance of this sarcoma on light microscopy required the use of immunohistochemistry and electron microscopy to establish the morphologic diagnosis and to exclude a diagnosis of other high-grade sarcoma, such as malignant fibrous histiocytoma or pleomorphic rhabdomyosarcoma. The observed clinical behavior of this leiomyosarcoma of the upper extremity seemed unusually aggressive compared with the biologic behavior predicted by the independent prognostic indicators of location, tumor size, and depth of invasion. The observed clinical behavior did not seem unusual compared with the clinical behavior predicted by the use of a staging scheme for soft tissue sarcomas.