A comprehensive statistical analysis of relapses to lung and abdomen and of deaths due to tumor that occurred among 1466 patients with nonmetastatic, favorable histology (FH) Wilms' tumor entered on the Third National Wilms' Tumor Study (NWTS-3) was undertaken. In addition to lymph node involvement, age at diagnosis and tumor size as measured by the weight of the excised specimen were the most important determinants of outcome. The effects of tumor size were most apparent for patients with localized (Stage I) disease; age effects were found for patients with regional (Stage II or III) disease. The presence of microscopic tumor in the margin of surgical resection was an independent predictor of abdominal recurrence and death in the latter group. Although the report of the surgeon of diffuse soilage of the peritoneal cavity from tumor spilled or cut across in the course of nephrectomy was highly correlated with outcome, it was not possible to establish an independent prognostic role for such a finding after adjustment for the aforementioned factors. Patients with intralobar nephrogenic rests (ILNR) had a favorable survival outlook even after accounting for their generally younger ages and lower stages. Cancer 68:2345–2353, 1991.