Multivariate statistical methods were used to study prognosis for 632 patients entered on the second National Wilms' Tumor Study who had nonmetastatic, unilateral disease at diagnosis. Separate analyses were conducted for each of four endpoints: abdominal recurrence, distant metastasis, relapse without regard to site, and death. The two most important predictors for metastasis and general relapse were an unfavorable (anaplastic or sarcomatous) histology and the presence of microscopically confirmed disease in the regional lymph nodes. Operative spillage of tumor increased the rates of abdominal recurrence and death, even after accounting for histology and lymph node effects. The presence of a tumor thrombus in the renal vein or IVC increased the risk of metastasis, and intrarenal vascular invasion was associated with general relapse after accounting for histology, lymph nodes, and spillage. However, these latter associations were weaker, and some uncertainty remains regarding the true prognostic import of such findings due to a high degree of collinearity among variables. By contrast to the results of a similar data analysis for the first National Wilms' Tumor Study, specimen weight had no bearing on outcome, and the effects of age at diagnosis were entirely explained by the association of age with other more critical factors.