Despite institutional variation in the treatment of synchronous bilateral Wilms tumor, a 76 per cent over-all 3-year survival rate has been achieved. We reviewed 145 patients with stage V disease (synchronous bilateral Wilms tumor) registered with National Wilms Tumor Studies 2 and 3. The majority of the patients (94, or 65 per cent) underwent initial surgical resection followed by chemotherapy with or without radiation, with a 3-year survival rate of 82 per cent. However, survival of 45 patients who underwent initial biopsy, postoperative chemotherapy and second-look surgery was 57 per cent at 3 years, a result not significantly different statistically from the 82 per cent figure. Only 38 per cent of the patients had all tumor resected at 1 or more operations. Therefore, emphasis is being placed on chemotherapy as the predominant treatment modality for this disease. Unfavorable histology was found in 10 per cent of the patients and 6 patients (4 per cent) had discordant histology, that is unfavorable histology on 1 side with contralateral favorable histology. Better prognostic factors were patient age less than 3 years at diagnosis, lower stage of the most advanced lesions, favorable histology and negative nodal involvement. Currently, routine bilateral biopsy of tumors is recommended, followed by chemotherapy according to the stage of the most advanced lesion. Impressive shrinkage of tumor can be expected with the hope that surgery will allow renal preservation (excisional biopsy or partial nephrectomy) rather than nephrectomy.