Combined modality treatment has resulted in cure rates of approximately 80% for children with Wilms' tumor. According to the National Wilms' Tumor Studies (NWTS), a group of patients with histologic features of anaplasia or sarcomatous Wilms' tumors (malignant rhabdoid tumors and clear cell sarcomas) were less responsive to vincristine and actinomycin. The survival rate of patients in this group with unfavorable histologic conditions was 54% compared with 90% for those with favorable histologic conditions. We have reviewed 80 consecutive cases of Wilms' tumor treated with a minimum follow-up interval of 5 years. Two pathologists independently reviewed all histologic specimens that were initially classified as having unfavorable histologic conditions and specimens from children with favorable histologic conditions who subsequently relapsed. One of 13 children with favorable histologic conditions had recurrent disease that was found to have unfavorable histologic conditions on rereview. All five patients with sarcomatous Wilms' tumor had a rapidly progressive course. Treatment of eight children with anaplastic Wilms' tumor with vincristine, actinomycin, cyclophosphamide, and abdominal radiation resulted in good disease-free and overall survival rates (5-year survival rate, 87.5%) that were not significantly different from children with tumors having favorable histologic conditions (5-year survival rate, 94%). All children with sarcomatous histologic conditions, however, did not to respond.