This study establishes the difficulty in making specific diagnoses in children with retroperitoneal tumors when only excretory urograms are used in diagnosis. Complementary use of excretory urography and ultrasonography should make the number of preoperative errors less by separating cases of Wilms tumor from those with cystic disease. This investigation underscores the hazards inherent in the use of preoperative irradiation and chemotherapy, since some children with benign disease are likely to receive inappropriate therapy.