UPJ Obstruction (Ureteropelvic Junction Obstruction): What It Is

Name: What Is UPJ Obstruction?
Creator: Prof. Dr. Ali Avanoğlu
Published: 2026-08-19

UPJ obstruction (ureteropelvic junction obstruction; informally, kidney outlet narrowing) is a narrowing at the point where the renal pelvis opens into the ureter. Urine passes through this point slowly, the pelvis widens, and this appears on ultrasound as hydronephrosis (kidney swelling). In most children it is congenital, and a large share can be followed without surgery.

Who this is forFamilies of children whose ultrasound report reads "UPJ obstruction" or "kidney outlet narrowing," and adults hearing this diagnosis for the first time

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ByProf. Dr. Ali AvanoğluUpdated19 August 2026

This page is part of UPJ obstruction.Read the full treatment overview on the hub page

kidney–ureter–bladder: SFU 3 hydronephrosis, narrowing at the UPJ (red ring)

01 · Where the kidney's outlet is, and what it does 02 · What UPJ obstruction actually means 03 · Who it affects, and how often 04 · How it is recognized 05 · Does every case of UPJ obstruction mean surgery?

In brief5 madde

Where the kidney's outlet is, and what it does: The kidney sends the urine it filters first into small collecting channels inside it, then into the renal pelvis.
What UPJ obstruction actually means: UPJ obstruction means the passage at the junction is narrow or non-functional enough to slow the flow of urine.
Who it affects, and how often: UPJ obstruction is reported in the literature at roughly 1 in 1,000–1,500 live births.
How it is recognized: The first step is always a kidney and bladder ultrasound.
Does every case of UPJ obstruction mean surgery?: No.

Where the kidney's outlet is, and what it does

The kidney sends the urine it filters first into small collecting channels inside it, then into the renal pelvis. The pelvis is funnel-shaped, and its lower end opens into the ureter — the thin tube that connects the kidney to the bladder.

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This transition point, where the pelvis opens into the ureter, is called the ureteropelvic junction (UPJ; PUJ in British sources). In a healthy junction, urine is carried downward continuously by wave-like contractions of the pelvis and ureter.
If the junction contains a segment that is narrow or does not contract properly, urine waits there. The first result of this waiting is that the pelvis widens; if it continues for a long time and is severe, pressure can build up on the kidney tissue.

What UPJ obstruction actually means

UPJ obstruction means the passage at the junction is narrow or non-functional enough to slow the flow of urine. The blockage is usually not complete — urine still passes, just more slowly than normal, and it backs up especially when urine output increases.

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This is why what shows up on ultrasound is not the narrowing itself but its consequence: hydronephrosis (widening of the kidney's collecting system with urine). Hydronephrosis is a finding, not a diagnosis; UPJ obstruction is one of its possible causes. This distinction matters at every stage of follow-up.
The degree of narrowing and its effect on the kidney vary widely from child to child. In some it stays silent and harmless for years; in others it can reduce kidney function. This is why the course over time is followed just as closely as the diagnosis itself.

Who it affects, and how often

UPJ obstruction is reported in the literature at roughly 1 in 1,000–1,500 live births. It is about twice as common in boys as in girls, and more common on the left side.

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In the newborn period, both kidneys are affected together in roughly 10–30% of cases; bilateral cases are followed more closely. Today, most children are noticed even before birth, through widening seen on a prenatal ultrasound.
The narrowing can be congenital, or it can appear later — in adolescents and adults — with pain or a stone. In the years before prenatal ultrasound was common, diagnosis was usually made at these later ages.

How it is recognized

The first step is always a kidney and bladder ultrasound. Ultrasound shows the front-to-back width of the pelvis (AP diameter), the degree of widening (SFU or UTD classification), and whether the ureter itself is widened; a normal ureter points to the narrowing being at the kidney's outlet.

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If the widening is significant, or increases during follow-up, MAG3 diuretic renography (a kidney scan) is performed. This test measures each kidney's share of total function (split function) and how quickly the pelvis empties after a diuretic is given.
If kidney reflux (vesicoureteral reflux, VUR) is suspected, the path diverges, and VCUG (a voiding study, informally the "catheter X-ray") comes into the picture.

Does every case of UPJ obstruction mean surgery?

No. Most children with UPJ-type widening found before birth are followed without surgery; the literature reports that only about a quarter (20–30%) eventually need pyeloplasty (the repair operation for UPJ obstruction). Improvement or stabilization usually happens in the first 2–3 years.

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The decision for surgery is not based on a single number; it is based on the widening's course over time, kidney function, drainage speed and symptoms together. When surgery is needed, Anderson–Hynes (dismembered) pyeloplasty is performed with a high success rate.
In our practice, two pediatric urology specialists carry this same process together from diagnosis through follow-up; the decision about which child is followed and which is operated on is made jointly.

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Frequently asked questions

Is UPJ obstruction the same thing as kidney outlet narrowing?

Yes. Ureteropelvic junction obstruction, UPJ obstruction, PUJ obstruction, and the everyday phrase "kidney outlet narrowing" all describe the same condition. You may also see it written as "UP junction obstruction" or "pelvi-ureteric narrowing" in reports.

Does UPJ obstruction go away on its own?

The narrowing itself is usually permanent; but in infants, as the junction grows, the passage can ease, and the widening can lessen or stay stable. For this reason, children with good function and no symptoms are followed with regular ultrasound, and most do not need surgery.

Can UPJ obstruction damage the kidney?

Mild, stable narrowing usually causes no harm. Severe or worsening narrowing can, over time, thin the kidney tissue under prolonged pressure and lead to loss of function; the goal of follow-up is to decide before that point is reached.

Does UPJ obstruction occur in adults too?

Yes. A congenital narrowing can remain silent for years and then appear in adolescence or adulthood as flank pain, a stone, or infection. The treatment principle is the same in adults; in our practice, the same specialists carry a patient through the transition from adolescence to adulthood.

Prof. Dr. Ali Avanoğlu's publications on this topic · 1

Avanoglu A, Tiryaki S (2020). Embryology and Morphological (Mal)Development of UPJ. Frontiers in pediatrics. PubMed ↗

Next • BasicsWhat Is Kidney Swelling (Hydronephrosis)→

Related pagesFull index →

BasicsWhat Is Kidney Swelling (Hydronephrosis)Kidney swelling (hydronephrosis) is a finding, not a diagnosis; UPJ obstruction is only one of its possible causes. In most babies whose ultrasound shows widening, the cause is temporary and needs no treatment at all. This page explains how we tell the two apart.BasicsWhy Does UPJ Obstruction HappenUPJ obstruction (ureteropelvic junction obstruction) is most often congenital, arising from a narrow or poorly contracting segment in the wall of the junction itself. In older children and adolescents, a crossing vessel compressing the junction from outside becomes more prominent; high ureteral insertion and acquired causes are rarer. The cause shapes the surgical plan.BasicsSymptoms of UPJ ObstructionUPJ obstruction most often causes no symptoms at all in infants and is noticed through a prenatal ultrasound. In older children and adolescents, episodic flank pain, nausea and vomiting, febrile urinary tract infection, blood in the urine, or a stone can be the first clue. This page walks through what to watch for at each age.Pyeloplasty and approachesWhat Is Pyeloplasty? The UPJ Obstruction Surgery, Step by StepPyeloplasty is the operation that repairs the narrow junction (UPJ) between the renal pelvis and the ureter. This page explains why the operation is done, the steps from preparation to discharge, and the question families ask most: "how many surgeries are needed."

UPJ Obstruction topic clustersMain treatment page →

Basics 7 Diagnosis and evaluation 6 Follow-up or surgery? 7 Conservative follow-up 5 Pyeloplasty and approaches 7 After surgery and follow-up 6 By age, and special situations 7

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