The National Wilms' Tumor Study, initiated in 1969, tested competing treatment strategems for patients with tumors ranging from Group (Gp) I (tumors confined to the kidney and totally removed) to Gp IV (remote metastases present at diagnosis). Three hundred and fifty-nine of 606 registered patients were randomized in the trial. Gp I patients under 2 years of age fared well whether postoperative radiation therapy (RT) was or was not added to 15 months' maintenance actinomycin D (AMD). Their prognosis was better than that for older cohorts similarly treated, in whom the difference in relapse rates between treatment groups were suggestive of an RT effect. Combined AMD and vincristine (VCR) gave better results than either agent alone in patients with more advanced tumors (Gps II and III) still confined to the abdomen, all of whom received postoperative RT as well. Preoperative VCR given Gp IV patients in addition to postoperative RT, AMD, and VCR did not improve results. The frequency of mesoblastic nephroma (1%), of bilateral tumors (5%), and of incorrect preoperative diagnosis of Wilms' tumor (5%), the toxicities of the various regimens, and other ancillary data are presented and discussed.