Forty-eight, computed tomography (CT) examinations undertaken in 30 patients with osteosarcoma of long bones were studied in detail, their diagnostic information being compared with that obtained from corresponding plain films. The latter were of more value in assessing peripheral bony involvement, by cortical extension and periosteal reaction, while the former, in general, permitted more accurate observation of extensions into adjacent soft tissues. Recognition of such extensions, however, was vitiated when they arose in relation to the proximal ends of the tibia, fibula, and humerus and when a haematoma had developed as a result of a biopsy. Plain films were also of more value in appreciation of response to chemotherapy. On the other hand CT is the only examination which permits a satisfactory study of intramedullary extensions of the tumour and in consequence is invaluable in determining the exact sites required for local resection. No cases of skip metastases were observed in our series, although similar appearances due to nutrient vessels or bony ridges, remote from the primary tumour, were noted on several occasions. Differentiation of these densities proved to be easy, particularly in the case of nutrient vessels when examination of the contralateral bone showed them to be symmetrical. Several authors have described the role of CT in the evaluation of local extensions. This paper reports the difficulties and errors encountered by us.