PURPOSE: We reviewed the long-term local tumor control in patients with bilateral Wilm's tumor (BWT) who received no definitive surgical therapy to one kidney after complete radiographic resolution after initial chemotherapy. METHODS: National Wilm's Tumor Study 4 (NWTS-4) enrolled 3335 patients (pts) during the period August 1986 to August 1994. There were 188 pts with BWT or 5.6% of the total enrolled. The treatment records and imaging reports were reviewed to ascertain those children who had documented tumors without definitive surgical therapy after initial treatment. Patients who did not have renal surgery because of progression of tumor were excluded from this study. RESULTS: Eleven children had no definitive surgical treatment of renal lesions in one kidney (right, 6; left, 5) after initial treatment with chemotherapy and/or radiation therapy. The pretreatment size of the lesions were less than 3 cm (4 pts), 3 to 6 cm (5 pts), more than 6 cm (1 pt), and unknown (1 pt). Prechemotherapy biopsy was performed in 6 of 11 patients. Lesions were less than 3 cm (1 pt), 3 to 6 cm (3 pts), more than 6 cm (1 pt), and unknown (1 pt). Four biopsy specimens showed favorable Wilm's histologic findings. One lesion (4 cm) showed an intralobar nephrogenic rest, another lesion of unknown size was read as favorable histologic findings vs perilobar nephrogenic rest. Biopsy was not performed on 5 lesions (4 pts, <1 cm; 1 pt, 3cm). Only 2 children in this study received radiation treatment. One child received 1050 cGy whole abdominal radiation, and 1 child received 1060 cGy to the left flank postnephrectomy. Radiation therapy was not given to any patient because of failure of the tumor to respond to chemotherapy. Five patients received treatment regimen EE-4A, dactinomycin, and vincristine. The duration of therapy ranged from 24 to 102 weeks for an average of 55.6 weeks. Three patients received treatment regimen DD-4A, dactinomycin, vincristine, and doxorubicin for 28, 52, and 52 weeks, respectively. Three patients received 2 separate regimens of chemotherapy. One child was treated with dactinomycin, vincristine, and cyclophosphamide for 60 weeks and then received regimen EE-4A for 24 weeks. Another patient received regimen EE-4A for 16 weeks and then regimen DD4-A for 36 weeks. One child received regimen EE-4-A for 12 weeks and then regimen DD4A for 40 weeks. Management of the contralateral kidney was complete nephrectomy in all 11 patients. There were no local relapses in the renal tumor bed. All patients were alive at a median follow-up of 9 years (range, 9 months to 15 years). CONCLUSION: Children with synchronous BWT or Wilm's tumor and contralateral nephrogenic rests that have radiographic resolution, after initial treatment, have a low risk for local relapse. These children should be followed by serial imaging.