The preoperative distinction between congenital renal cysts and malignant tumors in the newborn is often a difficult and important urographic problem. In the adult, the comparable situation has become significantly simplified with the advent and wide application of renal angiography and nephrotomography, both of which are impractical in the neonate. The nephrotomographic diagnosis of renal cysts in adults is based on the absolute avascularity of the renal mass in question in juxtaposition to opacified normal renal tissue. In 1963, O'Connor and Neuhauser (1) first applied the concept of total-body opacification to diagnostic radiology. They described this phenomenon in relation to conventional and high-dose urography in infancy. It was noted that in the course of intravenous urography in infants, particularly if a double dose (3–4 cc/kg) were given, the liver, spleen, bowel wall, heart, and virtually the entire blood vascular compartment opacified, enabling them to recognize the presence of relatively avascular areas. With this method, a cyst of the small bowel wall and a cavernous hemangioma of the liver with diffuse intratumoral thrombosis and hemorrhage were diagnosed. Following is the report of a case in which we were able to diagnose a congenital renal cyst in an infant preoperatively by applying this concept of total-body opacification. Case Report A one-day-old white female infant was transferred to The Long Island College Hospital from another because of a right abdominal mass present since birth. This had been found on routine postnatal physical examination. The patient was the product of a normal spontaneous delivery following an uneventful pregnancy. No other masses were noted. There was no significant family history of renal disease. Physical examination revealed an active female infant with no evidence of external anomaly. A slight subconjunctival hemorrhage was seen at the nasal aspect of the left bulbar conjunctiva. A visible bulge which moved with respiration was observed in the right abdomen. The mass was non-tender and nodular to palpation. The remainder of the physical findings were within normal limits. The working diagnosis on admission was Wilms's tumor. Roentgen Findings: On March 5, 1964, intravenous urography was performed, employing 10 cc of 50 per cent diatrizoate sodium. A scout film revealed a large mass occupying the entire right abdomen, displacing all visualized small bowel to the left. No calcifications were observed. The first injection, which was the equivalent of 2.5 cc/kg, failed to produce visualization of the right collecting system, despite delayed films taken after thirty minutes. Consequently, a second dose of 8 cc (total 4.5 cc/kg) was injected. This revealed a dilated collecting system with a markedly dilated and tortuous right ureter.