Ureteropelvic junction (UPJ) obstruction is generally a congenital (present from before birth) condition caused by narrowing of the connection between the ureter and the renal pelvis, which is part of the kidney. This blockage causes urine to build up in the renal pelvis, damaging the kidney.

UPJ obstruction is the most frequently diagnosed cause of urinary obstruction in children. It is now commonly diagnosed during prenatal ultrasound studies that show a dilated renal pelvis or a condition called hydronephrosis.

When recognized before the baby is born, UPJ obstruction may require sugical correction in the first few days after birth. Less severe cases may not require surgery until later in life, and some cases do not require surgery at all.

UPJ obstruction may be recognized after birth when an abdominal mass is found on examination, or if the infant develops a urinary tract infection associated with fever. Back pain and blood in the urine may also be signs of UPJ obstruction.