Robert L. Lebowitz, M.D. Children' s Hospital, Harvard Medical School Boston, Massachusetts

10 1/2 month-old infant boy who had prenatal diagnosis of right hydronephrosis.   VCUG normal.   Postnatal evaluation showed giant hydronephrosis on the right due to obstruction at the UPJ (Figure 1).   The right renal pelvis crossed the midline and touched the normal left renal pelvis, and extended from the renal fossa down into the patient's pelvis.   An IVP showed amazingly good concentrating ability.   Calyceal crescents were seen (Figure 2).  These are a sign of chronic obstructive hydronephrosis and show that concentrating ability has been preserved. The baby underwent dismembered pyeloplasty, and the post-op IVP (Figure 3) showed marked improvement in the degree of hydronephrosis and in the appearance of the contrast material in the collecting system.