A statistical analysis of 248 histologically proven cases of Wilms' tumor treated at the Institut Gustave-Roussy, Villejuif, France, from 1952 to 1967, is presented. Two years recurrence-free survival and five years survival have been selected as prognostic criteria. Patients under two years of age have significantly fewer metastases and a better five-year survival rate than those over two. Stage is stronly correlated with recurrences and survival. Influence of tumor size, number of tumor nodules, tumor rupture, adhesions, regional lymph node involvement, and renal vein infiltration is shown. Detailed analysis of histopathologic features indicates that a favorable clinical course is significantly related to the number of different varieties of epithelial differentiation (tubular, glomerular, microcystic) found in any tumor, whatever the abundance of each of them. Presence of striated muscular cells, however abundant, does not influence survival or metastatic rates. Cases where irradiation given prior to surgery resulted in complete destruction of tumor cells, form a distinctive group of very unfavorable prognosis. Relapse of tumor occurs in 54% of cases, 10% presenting with metastases. Involved sites are listed. No correlation is found between date of metastases and other parameters. The three major prognostic factors of age, stage, histologic pattern, bear prognostic value separately. Stage and age are linked, as well as histology and age. Stage and histology are not linked. Age in itself therefore seems less important. It is concluded that these well-defining treatment for each individual case.