The accurate assessment of the response of preoperative chemotherapy to osteosarcoma is very important not only for operative planing but for the post-operative treatment. Various imaging techniques have been evaluated in order to assess the reponse of preoperative chemotherapy to osteosarcoma, including conventional radiography, C.T., angiography, scintigraphy and MRI. Between August 1990 and March 1991, we treated 4 patients who had osteosarcoma at the proximal tibia. We compared the pre-chemotherapeutic and post-chemotherapeutic MR images and the grade of pathologic response. The results obtained from this study were as follows. 1. Increase in non-enhanced area ratio indicated good or fair pathologic response. 2. Non-enhanced area of Gd-DTPA enhanced MRI was corresponded to the necrotic area of the tumor mass. 3. Tumor volume ratio of the intraosseous component was not correlated to the grade of pathologic response. 4. MRI was non invasive, quantitative and the most accurate method for the assessment of response of the preoperative chemotherapy.