The National Wilms' Tumor Study 3 (NWTS-3) recommended treatment of bilateral Wilms' tumor with initial biopsy followed by chemotherapy with subsequent operation to resect the remaining tumor. This study was performed to determine if this approach preserves renal mass and function when compared with initial surgical resection followed by chemotherapy. Over a 20-year period (1970 to 1990), 15 patients with synchronous bilateral Wilms' tumor were treated at the Childrens Hospital of Los Angeles. Eight patients in the surgical group underwent initial unilateral nephrectomy with contralateral biopsy, wedge resection, or partial nephrectomy and subsequent chemotherapy. The seven patients in the chemotherapy group underwent bilateral tumor biopsy, followed by chemotherapy and subsequent tumor resection. Patients were assigned to each group in a nonrandomized manner according to the preference of the attending oncologist and surgeon. Comparison of the two groups showed no significant differences in sex distribution, initial renal function, tumor histology, dose and field of radiotherapy, presence or absence of positive surgical margins, and local recurrence rates. Patients in the surgery group were slightly older than those in the chemotherapy group: 3.6 +/- 2.2 versus 2.3 +/- 2.2 years. The percent of renal mass involved by tumor for the surgery group was 52 +/- 12 versus 73 +/- 16 for the chemotherapy group (P = .03). The percent of renal mass preserved following all operations, local recurrence rates, incidence of metastases, and survival was nearly identical between the two groups. There were three cases of renal failure in the surgery group and one case of renal failure in the chemotherapy group.(ABSTRACT TRUNCATED AT 250 WORDS)