The results of a controlled clinical trial of preoperative radiotherapy compared to chemotherapy in patients with nephroblastoma are presented. Of 397 histologically proven cases of Wilms' tumor registered at 34 centers between January 1977 and July 1979, 164 were eligible for the trial and were randomized to receive preoperative radiotherapy and chemotherapy (group R, 76 patients) or preoperative chemotherapy (group C, 88 patients). The results were evaluated in terms of the number of surgical tumor ruptures and of local tumor extent at pathologic examination, reflecting the effectiveness of the preoperative treatment. Survival and recurrence-free survival in the two treatment groups were also taken into account. The stage distribution was comparable in the two groups, with 52% stage I tumors in group R, and 43% in group C. Significant changes in the pathologic pattern were more frequent in group R than in group C (53% versus 17%). From these data it is concluded that preoperative chemotherapy is as good as preoperative radiotherapy in terms of prevention of tumor rupture. In addition, it was shown that 43% of an unselected population of patients with Wilms' tumor could be treated without any radiotherapy when chemotherapy had been given preoperatively.