Pelvi-ureteric junction (PUJ) obstruction is one of the causes of an obstructive uropathy which may be congenital or acquired. Ureteropelvic junction (UPJ) obstruction is a blockage at the point where ureters attaches to the bladder. This blocks the flow of urine out of the kidney, Urine can build up and damage the kidney, so radiologic imaging is crucial in diagnosing UPJ obstruction. It seemed therefore interesting to determine if some US parameters could predict the radionuclide parameters and which ultrasound parameter most influencing the kidney function. The initial step was to grade the severity of hydronephrosis, calyceal dilatation and cortical thinning, although it was easy for us to classify the importance of hydronephrosis in three groups according to anteroposterior diameter (APD). AP diameter of renal pelvis and differential renal function were the most effective parameters for surgical decision. These parameters can be used for appropriate management of antenatal hydronephrosis.