Patients are assigned to a treatment group based on initial therapy and histology. Patients under age 2 at diagnosis who were previously treated with nephrectomy as initial therapy for stage I favorable histology Wilms' tumor weighing less than 550 grams are assigned to group A. Patients who received Regimen EE-4A as initial therapy for Wilms' tumor are assigned to group B. Patients who received Regimen DD-4A as initial therapy for Wilms' tumor are assigned to group C. Patients with clear cell sarcoma of the kidney or diffuse anaplastic Wilms' tumor are assigned to group D.