Seventy-four patients who had a Ewing sarcoma of bone were managed with preoperative and postoperative chemotherapy and operative resection, with or without postoperative irradiation. The primary objectives of the study were to determine the histological response to preoperative chemotherapy in terms of the percentage of tumor necrosis and to assess the relationship between the histological response and the oncological outcome.The minimum duration of follow-up of the surviving patients who were continuously free of disease was five years. Sections of each operative specimen were examined, and the histological response to chemotherapy was graded semiquantitatively. Grade I indicated necrosis of 50 per cent of the tumor or less; grade II, necrosis of more than 50 per cent but less than 90 per cent; grade III, necrosis of 90 to 99 per cent; and grade IV, necrosis of 100 per cent of the tumor. Of the seventy-four tumors, forty-four (59 per cent) were exquisitely sensitive to chemotherapy and had complete (grade-IV) or nearly complete (grade-III) necrosis. In contrast, fourteen tumors (19 per cent) had little or no response to chemotherapy (grade I) and sixteen (22 per cent) had a moderate degree of necrosis (grade II).The histological response to preoperative chemotherapy (p = 0.0001), followed by the size of the tumor (p = 0.001), were the most important predictors of event-free survival. At five years, the rate of event-free survival was zero of fourteen patients who had had a grade-I response, six of sixteen who had had a grade-II response, and thirty-seven (84 per cent) of forty-four who had had a grade-III or IV response. The risk of local recurrence was most strongly associated with the operative margins; there were only four local recurrences (6 per cent) after sixty-seven resections with negative margins. Local recurrence may also have been influenced by the histological response and the use of local radiation. There were no local recurrences after operative treatment of six tumors that had been associated with pathological fracture.The histological response to preoperative chemotherapy and the size of the primary tumor are the most important clinical predictors of the outcome of operative treatment of non-metastatic Ewing sarcoma. These indicators should be used to identify patients who are at high risk for metastasis as such patients may be candidates for more intensive or novel therapies.