Eight cases of well-differentiated, intramedullary osteosarcoma were studied. In three cases, the lesions previously had been diagnosed, pathologically and radiologically, as benign. The distal femur was the most frequent site. Radiographically, the tumor margins were poorly defined in five cases and were well defined in three. In seven cases, the bone response pattern was both lytic and blastic; in one case, it was purely lytic. Tumor matrix patterns were mostly homogeneous and cloudlike. Four of the lesions were expansile. All cases had cortical thinning and discontinuity. Extraosseous involvement of the soft tissues was present in six cases, and active periosteal response was present in two. High-grade malignancies subsequently developed in two patients--one was a local recurrence and the other was a distant metastasis. A broad spectrum of radiologic features was observed in well-differentiated intramedullary osteosarcomas, but all cases had at least two of three radiographic findings (cortical discontinuity, poorly marginated soft-tissue extension, or cloudlike tumor matrix pattern) that strongly suggested malignancy.