MR imaging was used to monitor the results of initial chemotherapy of primary Ewing sarcoma of bone. The signal intensities of the soft-tissue and marrow components of the tumor were evaluated on T2-weighted images obtained in 10 patients (nine with responsive tumors) at presentation and during and immediately after completion of two cycles of chemotherapy. MR evidence of marrow and soft-tissue involvement was seen in all tumors at presentation. After treatment, the bone-marrow component of the nine drug-sensitive tumors showed an increase in signal intensity that in eight cases became comparable to that of water. Changes in signal intensity of the soft-tissue component were variable, consisting of increases in two of the responsive lesions, no change in three, a decrease in two, and complete resolution of the soft-tissue mass in two. There was no increase in signal intensity of either the bone-marrow or the soft-tissue component of the single nonresponsive tumor. All of the responsive tumors showed advanced healing, and abundant bony sclerosis was apparent on CT. Bone-marrow examinations, performed in seven of the nine patients with responsive lesions, disclosed no evidence of tumor in four. Two patients had residual extramedullary tumor; the nonresponsive lesion contained sheets of tumor cells. The increase in marrow signal intensity on T2-weighted images was associated with replacement of marrow elements by a loose, hypocellular myxoid matrix containing modest amounts of collagen, consistent with response to chemotherapy and eradication of disease. Therefore, an increase in the T2-weighted signal intensity of the bone-marrow component of Ewing sarcoma of bone reflected a favorable response to chemotherapy. MR signal changes, however, were not predictive of resolution of malignant disease within adjacent soft tissue.