The management of neonatal urinary tract dilatations represents one of the most challenging dilemmas in pediatric urology today. We have been confronted with 44 renal units in 35 neonates diagnosed as having primary obstructive megaureter during the last 6 years. Of these units 23 in 17 infants were diagnosed antenatally and 20 (87 per cent) have been managed without surgical intervention. Notably, 16 renal units were graded as moderate to severe megaureters by an excretory urogram. The decision to manage conservatively was based on the initial extraction of the 99mdiethylenetriaminepentaacetic acid renal scan (the extraction factor). This estimate of absolute renal function has been used to differentiate dilatations with obstructive implications for the renal parenchyma from those without. Significantly, expectant treatment has resulted in improvement of dilatation on sequential excretory urograms in 15 megaureters and none has shown a deterioration of function by renal scan. Similar diagnostic criterion also has resulted in conservative management for 12 of 21 additional neonatal megaureters seen during this period with symptoms or they were discovered serendipitously. Only 2 of these 12 megaureters required surgical correction. The neonatal primary megaureter appears in many cases to represent a different entity than those that commonly presented before the advent of antenatal and perinatal diagnosis.