The uretero-pelvic junction is the junction between the ureter

Ureter

In human anatomy, the ureters are muscular tubes that propel urine from the kidneys to the urinary bladder. In the adult, the ureters are usually long and ~3-4 mm in diameter....

 and the renal pelvis

Renal pelvis

The renal pelvis or pyelum is the funnel-like dilated proximal part of the ureter in the kidney.In humans, the renal pelvis is the point of convergence of two or three major calyces...

 of the kidney

Kidney

The kidneys, organs with several functions, serve essential regulatory roles in most animals, including vertebrates and some invertebrates. They are essential in the urinary system and also serve homeostatic functions such as the regulation of electrolytes, maintenance of acid–base balance, and...

.

During the embryologic development of the kidney, this is the last part of the ureter

Ureter

In human anatomy, the ureters are muscular tubes that propel urine from the kidneys to the urinary bladder. In the adult, the ureters are usually long and ~3-4 mm in diameter....

 to become patent.

Failure of this segment of ureter to become patent during development is the most frequent cause of bilateral hydronephrosis

Hydronephrosis

Hydronephrosis is distension and dilation of the renal pelvis calyces, usually caused by obstruction of the free flow of urine from the kidney, leading to progressive atrophy of the kidney...

, particularly in male neonates. Pyeloplasty

Pyeloplasty

Pyeloplasty is the surgical reconstruction or revision of the renal pelvis to drain and decompress the kidney. Most commonly it is performed to treat an uretero-pelvic junction obstruction if residual renal function is adequate....

, which involves excision of the stenotic section and creation of a new junction, is the most common and effective treatment for this problem.