We studied the CT and MR scans, and the histology of 50 patients with primary Ewing's sarcoma of bone to determine the association between the change in tumour volume and necrosis after chemotherapy, and to ascertain their influence on prognosis. The mean age of the patients was 17 years. The limbs were involved in 40 and the axial bones in ten. The volume of the tumour at diagnosis varied from 31 to 1790 ml. There was a significant relationship between necrosis and the measured change in volume of the tumour after chemotherapy. Progression of the tumour despite chemotherapy was seen only in patients with necrosis of grades 4 to 6. Necrosis significantly influenced survival (p < 0.05), but the effect of change in volume was less significant. Change in volume of the tumour is a good predictor of necrosis induced by chemotherapy. Necrosis is a strong prognostic factor in Ewing's sarcoma.