The term "teratoid" Wilms' tumor has been used recently to describe an unusual type of tumor in which, although classic nephroblastoma tissue is present, there is a significant diversity of cell types and tissues present. Examination of tissue samples from 290 patients treated at St Jude Children's Research Hospital from 1964 to 1987 disclosed that three children had teratoid Wilms' tumor. All three children had renal tumors and two of them presented with bilateral pyeloureteral obstruction, uremia, and hypertension. When compared with classic nephroblastoma, two of the children with teratoid elements responded poorly to chemotherapy and irradiation, although no metastatic disease was identified. One child died with sepsis and renal failure; the other two are surviving disease-free for 7 1/2 years and 26+ months since diagnosis. Because of the tendency for bilateral involvement, ureteral obstruction, and uremia, and their relative resistance to chemotherapy and irradiation, surgery is the principal form of therapy for patients with these tumors.