An analysis of 6 representative cases of tumors of the pelvis and lower extremities demonstrates the clinical usefulness of computerized tomography (CT) in preoperative evaluation. CT provides information regarding precise size of the mass, its location and relationship to muscle bundles and fascial planes, definition of the margins of the mass, relation to neural, vascular and bony structures not previously available by plain film and angiography. This information is of particular value to the orthopedic surgeon in evaluating the feasibility of en bloc resections of tumors and in planning reconstructive procedures.