Since the introduction of antenatal ultrasound, the age of patients presenting with pyelocaliceal dilatation has continued to decrease: before the era of ultrasound most patients with this diagnosis were symptomatic; today most patients are diagnosed before birth and most of them are without symptoms. The current diagnostic tools are diuretic renography and ultrasound, after vesicoureteral reflux has been excluded. The realization that obstructive renographic drainage curves were not reliable indicators of obstruction in this patient group eventually altered the therapeutic approach from early operative intervention to a more conservative management in patients with a good relative kidney function. Ultrasound has significant value in detection of patients at risk of loss of renal function. With this management only a quarter appear to have a significant ureteropelvic junction stenosis needing surgery.