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Objective: To provide guidelines for future trials, we reviewed the outcomes of children with synchronous bilateral Wilms tumors (BWT) treated on National Wilms Tumor Study-4 (NWTS-4)., Methods: NWTS-4 enrolled 3335 patients including 188 patients with BWT (5.6%). Treatment and outcome data were collected., Results: Among 188 BWT patients registered with NWTS-4, 195 kidneys in 123 patients had initial open biopsy, 44 kidneys in 31 patients had needle biopsies. Although pre-resection chemotherapy was recommended, 87 kidneys in 83 patients were managed with primary resection: Complete nephrectomy 48 in 48 patients, 31 partial/wedge nephrectomies in 27 patients, enucleations 8 in 8 patients. No initial surgery was performed in 45 kidneys in 43 patients, 5 kidneys in 3 patients not coded. Anaplasia was diagnosed after completion of the initial course of chemotherapy in 14 patients (initial surgical procedure: 9 open biopsies, 4 needle biopsies, 1 partial nephrectomy). The average number of days from the start of chemotherapy to diagnosis of anaplasia was 390 (range 44-1925 days). Relapse or progression of disease occurred in 54 children. End stage renal failure occurred in 23 children, 6 of whom had bilateral nephrectomies. The 8 year event free survival for BWT with favorable histology was 74%, and overall survival was 89%; whereas the event free survival for BWT with unfavorable histology was 40%, overall survival was 45%., Conclusion: The current analysis of patients with BWT treated on NWTS-4 shows that preservation of renal parenchyma is possible in many patients after initial preoperative chemotherapy. The incidence of end-stage renal disease remains significantly higher in children with BWT. Future studies are warranted to address the need for earlier biopsy in nonresponsive tumors and earlier definitive surgery to recognize unfavorable histology in these high-risk patients.