Parosteal osteosarcoma, a distinct entity in which the neoplasm arises on the bone surface, presents characteristic features. Thorough radiologic and histologic evaluation and early definitive surgery usually result in a favorable prognosis and make limb salvage feasible in many adult patients. Twenty-six patients with proven parosteal osteosarcoma were seen at The University of Texas M. D. Anderson Hospital and Tumor Institute at Houston. All were examined by conventional radiography, 16 by CT, and one by both CT and MR. CT is valuable for the evaluation of tumor invasion of the medullary canal, involvement of the cortex, and extension into the soft tissues, findings frequently not demonstrable by other noninvasive techniques. Additional information is obtained regarding the presence and location of satellite lesions and intralesional radiolucent areas and the relationship of the vascular bundle to the tumor mass. These findings are important for planning both percutaneous biopsy and surgery.