The Cooperative Ewing's Sarcoma Studies, CESS 81 and CESS 86, are multiinstitutional trials with more than 80 participating institutions from Germany, the Netherlands, Austria, and Switzerland. Treatment consists of four courses of multiagent chemotherapy and local therapy. Local therapy was not randomized and was either radical surgery or resection plus postoperative irradiation or definitive radiation therapy. Here results according to local therapy have been analyzed for 93 protocol patients with localized Ewing's sarcoma (ES) who have been recruited in CESS 81 from January 1981 to February 1985 and 122 protocol patients recruited in CESS 86 from January 1986 to November 1989. The 3-year relapse-free survival (RFS) was 55% in CESS 81 and 62% in CESS 86. In CESS 81, the RFS was better for surgically treated than for irradiated patients. In this study there was an extremely high incidence of local failures (50%) after definitive irradiation. In CESS 86, however, the results after radiation therapy have been improved markedly (3-year RFS 67% after radiation therapy, 65% after surgery, and 62% after resection plus irradiation). Possible explanations for the improvement of radiotherapeutic results are as follows: selections for patients for radiation therapy, start of local therapy, and quality of radiation therapy. In CESS 86, irradiated patients were randomized to receive either conventionally fractionated irradiation with less intense chemotherapy or hyperfractionated irradiation with simultaneous chemotherapy. There was no difference in treatment results at the time of analysis. The authors conclude that selection of patients for local treatment modalities and quality of treatment performance has an impact on local and overall treatment results in ES.