In a review of 47 cases of histologically proved Wilms's tumor encountered in the period 1941-1955, several factors emerged as relevant to the prognosis. Tumor thrombi in renal veins, capsular involvement, size of tumor, and percentage of renal tissue replaced by tumor influenced the chance of survival unfavorably. The most important factor was age. Nine out of 15 patients (67%) under 2 years of age, but only 4 out of 32 (13%) patients 2 years of age or older survived 5 years or more. Out of 21 patients who received preoperative radiation, nephrectomy and postoperative radiation, 12 (57%) survived 3 or more years. The excretory urographic examination was considered an adequate diagnostic tool in the majority of cases.