The treatment of bilateral Wilms tumor (BWT) involves a multidisciplinary approach including surgery, chemotherapy, and radiation therapy. The long-term renal function in patients receiving all three treatment modalities has not been evaluated. Long-term renal function was evaluated in 81 children with synchronous BWT who received radiation therapy as part of their treatment. Renal function was assessed by measuring blood urea nitrogen (BUN) and serum creatinine (Cr). The normal range for the BUN was defined as 10-24 mg/dl, and the Cr was considered normal at levels of <1.5 mg/dl. Moderate elevations were defined as a BUN of 25-50 mg/dl and/or a Cr of 1.6-2.5 mg/dl and marked elevations as a BUN of >50 mg/dl and/or a Cr of >2.5 mg/dl. BUN and Cr levels were measured prior to treatment and at the following intervals: 6 months after treatment, 1 year after treatment, 2 years after treatment, and at last follow-up. Any elevation during the posttreatment follow-up period was considered abnormal. A total of 28 children (34.6%) had elevated BUN and/or Cr levels, and 18 had moderate and 10 had marked renal insufficiency. No dose-response relationship was established when comparing the radiation doses of those with elevated values to those with normal values. The renal complication rate was moderate, and other factors including surgery, extent and nature of chemotherapy, and recurrent tumor must also be taken into account. The elevations present in several children could be attributed to tumor recurrence and in one case to gentamicin toxicity. The management of children with BWT should consider all of these risks, and attempts to preserve renal parenchyma are warranted.