Surgical data derived from the 606 patients in the National Wilms' Tumor Study have been analyzed to determine the effect of surgical technique on results of treatment. In addition to surgical excision of the tumor, patients were treated with chemotherapy and radiation therapy according to the study protocol. Under these controlled conditions, certain aspects of surgical technique which have traditionally been thought to be important for success appear to be irrelevant. Physical characteristics of the tumor, preoperative rupture and vascular invasion by tumor were not associated with higher relapse rates. Large tumors, those with capsular infiltrations, and tumors with spread to lymph nodes higher recurrence rate. Operative spill increased the chance of abdominal recurrence. There was no evidence that early ligation of the renal vein was of value in prevention of recurrence, nor was incomplete removal of tumor associated with an increase in relapse rate. Although several critical factors of surgical technique were not studied, it is clear that others are not significant and need not be continued.