In 15 osteosarcomas and six Ewing sarcomas, response to preoperative chemotherapy was assessed with magnetic resonance (MR) imaging without and with gadolinium diethylenetriaminepentaacetic acid (DTPA) enhancement and with dynamic Gd-DTPA studies, and the results were compared with the scintigraphic findings. All studies were obtained prior to and following preoperative chemotherapy. Static MR imaging was of little value for assessment of response; reduction in signal intensity within soft-tissue masses on the T2-weighted spin-echo images indicated response with a sufficient degree of accuracy (71%) but low sensitivity, whereas an increase in signal intensity after Gd-DTPA administration indicated zones of viable tissue with low specificity. With three-phase skeletal scintigraphy, the findings in the perfusion and blood-pool phases were of no value, whereas the findings in the osseous phase allowed the prediction of response with an accuracy of 73.7%. Of all techniques employed, dynamic MR imaging had the highest degree of accuracy (85.7%) and was superior to scintigraphy, particularly in patients who were receiving intraarterial chemotherapy.