Ureteropelvic junction (UPJ) obstruction is a partial or total blockage at the place where the organ that produces urine (the kidney) and the tube that carries it to the bladder (the ureter) are joined. The kidney filters blood and collects urine at its center,
where the renal pelvis funnels it into the ureter so that it can be delivered to the bladder. More common in children than in adults and often resulting from a congenital abnormality, UPJ obstruction impedes the flow of urine, causing it to build up and resulting in hydronephrosis – an enlargement of the renal pelvis – and the potential for kidney damage.

Urinary Tract Illustration, UCLA UPJ obstruction is the most common cause of hydronephrosis detected on prenatal ultrasound or in newborns. The blockage can range from minimal to severe. Mild cases usually don’t damage the kidney or impair its function, but they can predispose the child to urinary tract infections. Severe cases can cause significant harm to the kidney.